Feat: 소셜 로그인 추가 및 인증 페이지 UI 테마 정리

app/login/page.tsx
- Google/Kakao 소셜 로그인 버튼을 폼으로 연동하고 액션 호출 추가
- 로그인 페이지의 배경/버튼/텍스트 색상 등 UI 테마를 파스텔에서 그레이/다크톤으로 통일
- 소셜 버튼 마크업 및 스타일 정리

app/signup/page.tsx
- 회원가입 페이지 배경 및 버튼 색상을 그레이/다크톤으로 변경
- 아이콘 배경 그라디언트 및 링크 색상 업데이트

app/forgot-password/page.tsx
- 비밀번호 재설정 요청 페이지의 배경, 블러 효과 및 버튼 스타일을 그레이/다크톤으로 변경
- 아이콘 배경 및 링크 색상 업데이트

app/reset-password/page.tsx
- 비밀번호 재설정 페이지의 배경, 블러 효과 및 버튼 스타일을 그레이/다크톤으로 변경
- 아이콘 배경 업데이트

features/auth/actions.ts
- Google 및 Kakao OAuth 시작을 위한 공통 헬퍼(signInWithProvider) 추가
- signInWithGoogle 및 signInWithKakao 액션을 구현하여 OAuth 흐름을 시작하도록 추가

app/forgot-password/page.tsx

@@ -29,14 +29,14 @@ export default async function ForgotPasswordPage({
   const { message } = await searchParams;
 
   return (
-    <div className="relative flex min-h-screen items-center justify-center overflow-hidden bg-gradient-to-br from-indigo-50 via-purple-50 to-pink-50 px-4 py-12 dark:from-gray-950 dark:via-indigo-950 dark:to-purple-950">
+    <div className="relative flex min-h-screen items-center justify-center overflow-hidden bg-gradient-to-br from-gray-50 via-white to-gray-100 px-4 py-12 dark:from-black dark:via-gray-950 dark:to-gray-900">
       {/* ========== 배경 그라디언트 ========== */}
-      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top_right,_var(--tw-gradient-stops))] from-indigo-200/40 via-purple-200/20 to-transparent dark:from-indigo-900/30 dark:via-purple-900/20" />
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top_right,_var(--tw-gradient-stops))] from-gray-200/30 via-gray-100/15 to-transparent dark:from-gray-800/30 dark:via-gray-900/20" />
-      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_bottom_left,_var(--tw-gradient-stops))] from-pink-200/40 via-purple-200/20 to-transparent dark:from-pink-900/30 dark:via-purple-900/20" />
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_bottom_left,_var(--tw-gradient-stops))] from-gray-300/30 via-gray-200/15 to-transparent dark:from-gray-700/30 dark:via-gray-800/20" />
 
       {/* ========== 애니메이션 블러 효과 ========== */}
-      <div className="absolute left-1/4 top-1/4 h-64 w-64 animate-pulse rounded-full bg-indigo-400/30 blur-3xl dark:bg-indigo-600/20" />
+      <div className="absolute left-1/4 top-1/4 h-64 w-64 animate-pulse rounded-full bg-gray-300/20 blur-3xl dark:bg-gray-700/20" />
-      <div className="absolute bottom-1/4 right-1/4 h-64 w-64 animate-pulse rounded-full bg-purple-400/30 blur-3xl delay-700 dark:bg-purple-600/20" />
+      <div className="absolute bottom-1/4 right-1/4 h-64 w-64 animate-pulse rounded-full bg-gray-400/20 blur-3xl delay-700 dark:bg-gray-600/20" />
 
       {/* ========== 메인 콘텐츠 ========== */}
       <div className="relative z-10 w-full max-w-md animate-in fade-in slide-in-from-bottom-4 duration-700">
@@ -47,7 +47,7 @@ export default async function ForgotPasswordPage({
         <Card className="border-white/20 bg-white/70 shadow-2xl backdrop-blur-xl dark:border-white/10 dark:bg-gray-900/70">
           <CardHeader className="space-y-3 text-center">
             {/* 아이콘 */}
-            <div className="mx-auto mb-2 flex h-16 w-16 items-center justify-center rounded-2xl bg-gradient-to-br from-indigo-500 to-purple-600 shadow-lg">
+            <div className="mx-auto mb-2 flex h-16 w-16 items-center justify-center rounded-2xl bg-gradient-to-br from-gray-800 to-black shadow-lg dark:from-white dark:to-gray-200">
               <span className="text-4xl">🔑</span>
             </div>
             {/* 페이지 제목 */}
@@ -85,7 +85,7 @@ export default async function ForgotPasswordPage({
               {/* ========== 재설정 링크 발송 버튼 ========== */}
               <Button
                 formAction={requestPasswordReset}
-                className="h-11 w-full bg-gradient-to-r from-indigo-600 to-purple-600 font-semibold text-white shadow-lg transition-all hover:from-indigo-700 hover:to-purple-700 hover:shadow-xl"
+                className="h-11 w-full bg-gradient-to-r from-gray-900 to-black font-semibold text-white shadow-lg transition-all hover:from-black hover:to-gray-800 hover:shadow-xl dark:from-white dark:to-gray-100 dark:text-black dark:hover:from-gray-100 dark:hover:to-white"
               >
                 재설정 링크 보내기
               </Button>
@@ -95,7 +95,7 @@ export default async function ForgotPasswordPage({
             <div className="text-center">
               <Link
                 href="/login"
-                className="text-sm font-medium text-indigo-600 hover:text-indigo-500 dark:text-indigo-400 dark:hover:text-indigo-300"
+                className="text-sm font-medium text-gray-700 hover:text-black dark:text-gray-300 dark:hover:text-white"
               >
                 ← 로그인 페이지로 돌아가기
               </Link>

app/login/page.tsx

@@ -1,5 +1,9 @@
 import Link from "next/link";
-import { login } from "@/features/auth/actions";
+import {
+  login,
+  signInWithGoogle,
+  signInWithKakao,
+} from "@/features/auth/actions";
 import FormMessage from "@/components/form-message";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
@@ -33,24 +37,24 @@ export default async function LoginPage({
   const { message } = await searchParams;
 
   return (
-    <div className="relative flex min-h-screen items-center justify-center overflow-hidden bg-gradient-to-br from-indigo-50 via-purple-50 to-pink-50 px-4 py-12 dark:from-gray-950 dark:via-indigo-950 dark:to-purple-950">
+    <div className="relative flex min-h-screen items-center justify-center overflow-hidden bg-gradient-to-br from-gray-50 via-white to-gray-100 px-4 py-12 dark:from-black dark:via-gray-950 dark:to-gray-900">
       {/* ========== 배경 그라디언트 레이어 ========== */}
       {/* 웹 페이지 전체 배경을 그라디언트로 채웁니다 */}
-      {/* 라이트 모드: 부드러운 파스텔 톤 (indigo → purple → pink) */}
+      {/* 라이트 모드: 부드러운 그레이 톤 (gray → white → gray) */}
-      {/* 다크 모드: 진한 어두운 톤으로 눈부심 방지 */}
+      {/* 다크 모드: 깊은 블랙 톤으로 고급스러운 느낌 */}
 
       {/* 추가 그라디언트 효과 1: 우상단에서 시작하는 원형 그라디언트 */}
-      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top_right,_var(--tw-gradient-stops))] from-indigo-200/40 via-purple-200/20 to-transparent dark:from-indigo-900/30 dark:via-purple-900/20" />
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top_right,_var(--tw-gradient-stops))] from-gray-200/30 via-gray-100/15 to-transparent dark:from-gray-800/30 dark:via-gray-900/20" />
 
       {/* 추가 그라디언트 효과 2: 좌하단에서 시작하는 원형 그라디언트 */}
-      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_bottom_left,_var(--tw-gradient-stops))] from-pink-200/40 via-purple-200/20 to-transparent dark:from-pink-900/30 dark:via-purple-900/20" />
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_bottom_left,_var(--tw-gradient-stops))] from-gray-300/30 via-gray-200/15 to-transparent dark:from-gray-700/30 dark:via-gray-800/20" />
 
       {/* ========== 애니메이션 블러 효과 ========== */}
       {/* 부드럽게 깜빡이는 원형 블러로 생동감 표현 */}
       {/* animate-pulse: 1.5초 주기로 opacity 변화 */}
-      <div className="absolute left-1/4 top-1/4 h-64 w-64 animate-pulse rounded-full bg-indigo-400/30 blur-3xl dark:bg-indigo-600/20" />
+      <div className="absolute left-1/4 top-1/4 h-64 w-64 animate-pulse rounded-full bg-gray-300/20 blur-3xl dark:bg-gray-700/20" />
       {/* delay-700: 700ms 지연으로 교차 애니메이션 효과 */}
-      <div className="absolute bottom-1/4 right-1/4 h-64 w-64 animate-pulse rounded-full bg-purple-400/30 blur-3xl delay-700 dark:bg-purple-600/20" />
+      <div className="absolute bottom-1/4 right-1/4 h-64 w-64 animate-pulse rounded-full bg-gray-400/20 blur-3xl delay-700 dark:bg-gray-600/20" />
 
       {/* ========== 메인 콘텐츠 영역 ========== */}
       {/* z-10: 배경보다 위에 표시 */}
@@ -67,7 +71,7 @@ export default async function LoginPage({
           {/* ========== 카드 헤더 영역 ========== */}
           <CardHeader className="space-y-3 text-center">
             {/* 아이콘 배경: 그라디언트 원형 */}
-            <div className="mx-auto mb-2 flex h-16 w-16 items-center justify-center rounded-2xl bg-gradient-to-br from-indigo-500 to-purple-600 shadow-lg">
+            <div className="mx-auto mb-2 flex h-16 w-16 items-center justify-center rounded-2xl bg-gradient-to-br from-gray-800 to-black shadow-lg dark:from-white dark:to-gray-200">
               <span className="text-4xl">👋</span>
             </div>
             {/* 페이지 제목 */}
@@ -138,7 +142,7 @@ export default async function LoginPage({
                 {/* 비밀번호 찾기 링크 */}
                 <Link
                   href="/forgot-password"
-                  className="text-sm font-medium text-indigo-600 hover:text-indigo-500 dark:text-indigo-400 dark:hover:text-indigo-300"
+                  className="text-sm font-medium text-gray-700 hover:text-black dark:text-gray-300 dark:hover:text-white"
                 >
                   비밀번호 찾기
                 </Link>
@@ -148,7 +152,7 @@ export default async function LoginPage({
               <Button
                 formAction={login}
                 type="submit"
-                className="h-11 w-full bg-gradient-to-r from-indigo-600 to-purple-600 font-semibold shadow-lg transition-all duration-200 hover:from-indigo-700 hover:to-purple-700 hover:shadow-xl"
+                className="h-11 w-full bg-gradient-to-r from-gray-900 to-black font-semibold shadow-lg transition-all duration-200 hover:from-black hover:to-gray-800 hover:shadow-xl dark:from-white dark:to-gray-100 dark:text-black dark:hover:from-gray-100 dark:hover:to-white"
                 size="lg"
               >
                 로그인
@@ -159,7 +163,7 @@ export default async function LoginPage({
                 계정이 없으신가요?{" "}
                 <Link
                   href="/signup"
-                  className="font-semibold text-indigo-600 transition-colors hover:text-indigo-500 dark:text-indigo-400 dark:hover:text-indigo-300"
+                  className="font-semibold text-gray-900 transition-colors hover:text-black dark:text-gray-100 dark:hover:text-white"
                 >
                   회원가입 하기
                 </Link>
@@ -176,45 +180,52 @@ export default async function LoginPage({
 
             {/* 소셜 로그인 버튼들 */}
             <div className="grid grid-cols-2 gap-3">
-              <Button
+              {/* ========== Google 로그인 버튼 ========== */}
-                type="button"
+              <form action={signInWithGoogle}>
-                variant="outline"
+                <Button
-                className="h-11 border-gray-200 bg-white shadow-sm transition-all duration-200 hover:bg-gray-50 hover:shadow-md dark:border-gray-700 dark:bg-gray-800 dark:hover:bg-gray-750"
+                  type="submit"
-              >
+                  variant="outline"
-                <svg className="mr-2 h-5 w-5" viewBox="0 0 24 24">
+                  className="h-11 w-full border-gray-200 bg-white shadow-sm transition-all duration-200 hover:bg-gray-50 hover:shadow-md dark:border-gray-700 dark:bg-gray-800 dark:hover:bg-gray-750"
-                  <path
-                    fill="currentColor"
-                    d="M22.56 12.25c0-.78-.07-1.53-.2-2.25H12v4.26h5.92c-.26 1.37-1.04 2.53-2.21 3.31v2.77h3.57c2.08-1.92 3.28-4.74 3.28-8.09z"
-                  />
-                  <path
-                    fill="currentColor"
-                    d="M12 23c2.97 0 5.46-.98 7.28-2.66l-3.57-2.77c-.98.66-2.23 1.06-3.71 1.06-2.86 0-5.29-1.93-6.16-4.53H2.18v2.84C3.99 20.53 7.7 23 12 23z"
-                  />
-                  <path
-                    fill="currentColor"
-                    d="M5.84 14.09c-.22-.66-.35-1.36-.35-2.09s.13-1.43.35-2.09V7.07H2.18C1.43 8.55 1 10.22 1 12s.43 3.45 1.18 4.93l2.85-2.22.81-.62z"
-                  />
-                  <path
-                    fill="currentColor"
-                    d="M12 5.38c1.62 0 3.06.56 4.21 1.64l3.15-3.15C17.45 2.09 14.97 1 12 1 7.7 1 3.99 3.47 2.18 7.07l3.66 2.84c.87-2.6 3.3-4.53 6.16-4.53z"
-                  />
-                </svg>
-                Google
-              </Button>
-              <Button
-                type="button"
-                variant="outline"
-                className="h-11 border-none bg-[#FEE500] font-semibold text-[#3C1E1E] shadow-sm transition-all duration-200 hover:bg-[#FDD835] hover:shadow-md"
-              >
-                <svg
-                  className="mr-2 h-5 w-5"
-                  viewBox="0 0 24 24"
-                  fill="currentColor"
                 >
-                  <path d="M12 3c-4.97 0-9 4.03-9 9s4.03 9 9 9 9-4.03 9-9-4.03-9-9-9zm-1.5 14.5h-3v-9h3v9zm3 0h-3v-5h3v5zm0-6h-3v-3h3v3z" />
+                  <svg className="mr-2 h-5 w-5" viewBox="0 0 24 24">
-                </svg>
+                    <path
-                Kakao
+                      fill="currentColor"
-              </Button>
+                      d="M22.56 12.25c0-.78-.07-1.53-.2-2.25H12v4.26h5.92c-.26 1.37-1.04 2.53-2.21 3.31v2.77h3.57c2.08-1.92 3.28-4.74 3.28-8.09z"
+                    />
+                    <path
+                      fill="currentColor"
+                      d="M12 23c2.97 0 5.46-.98 7.28-2.66l-3.57-2.77c-.98.66-2.23 1.06-3.71 1.06-2.86 0-5.29-1.93-6.16-4.53H2.18v2.84C3.99 20.53 7.7 23 12 23z"
+                    />
+                    <path
+                      fill="currentColor"
+                      d="M5.84 14.09c-.22-.66-.35-1.36-.35-2.09s.13-1.43.35-2.09V7.07H2.18C1.43 8.55 1 10.22 1 12s.43 3.45 1.18 4.93l2.85-2.22.81-.62z"
+                    />
+                    <path
+                      fill="currentColor"
+                      d="M12 5.38c1.62 0 3.06.56 4.21 1.64l3.15-3.15C17.45 2.09 14.97 1 12 1 7.7 1 3.99 3.47 2.18 7.07l3.66 2.84c.87-2.6 3.3-4.53 6.16-4.53z"
+                    />
+                  </svg>
+                  Google
+                </Button>
+              </form>
+
+              {/* ========== Kakao 로그인 버튼 ========== */}
+              <form action={signInWithKakao}>
+                <Button
+                  type="submit"
+                  variant="outline"
+                  className="h-11 w-full border-none bg-[#FEE500] font-semibold text-[#3C1E1E] shadow-sm transition-all duration-200 hover:bg-[#FDD835] hover:shadow-md"
+                >
+                  <svg
+                    className="mr-2 h-5 w-5"
+                    viewBox="0 0 24 24"
+                    fill="currentColor"
+                  >
+                    <path d="M12 3c-4.97 0-9 4.03-9 9s4.03 9 9 9 9-4.03 9-9-4.03-9-9-9zm-1.5 14.5h-3v-9h3v9zm3 0h-3v-5h3v5zm0-6h-3v-3h3v3z" />
+                  </svg>
+                  Kakao
+                </Button>
+              </form>
             </div>
           </CardContent>
         </Card>

app/reset-password/page.tsx

@@ -72,14 +72,14 @@ export default async function ResetPasswordPage({
   const { message } = params;
 
   return (
-    <div className="relative flex min-h-screen items-center justify-center overflow-hidden bg-gradient-to-br from-indigo-50 via-purple-50 to-pink-50 px-4 py-12 dark:from-gray-950 dark:via-indigo-950 dark:to-purple-950">
+    <div className="relative flex min-h-screen items-center justify-center overflow-hidden bg-gradient-to-br from-gray-50 via-white to-gray-100 px-4 py-12 dark:from-black dark:via-gray-950 dark:to-gray-900">
       {/* ========== 배경 그라디언트 ========== */}
-      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top_right,_var(--tw-gradient-stops))] from-indigo-200/40 via-purple-200/20 to-transparent dark:from-indigo-900/30 dark:via-purple-900/20" />
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top_right,_var(--tw-gradient-stops))] from-gray-200/30 via-gray-100/15 to-transparent dark:from-gray-800/30 dark:via-gray-900/20" />
-      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_bottom_left,_var(--tw-gradient-stops))] from-pink-200/40 via-purple-200/20 to-transparent dark:from-pink-900/30 dark:via-purple-900/20" />
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_bottom_left,_var(--tw-gradient-stops))] from-gray-300/30 via-gray-200/15 to-transparent dark:from-gray-700/30 dark:via-gray-800/20" />
 
       {/* ========== 애니메이션 블러 효과 ========== */}
-      <div className="absolute left-1/4 top-1/4 h-64 w-64 animate-pulse rounded-full bg-indigo-400/30 blur-3xl dark:bg-indigo-600/20" />
+      <div className="absolute left-1/4 top-1/4 h-64 w-64 animate-pulse rounded-full bg-gray-300/20 blur-3xl dark:bg-gray-700/20" />
-      <div className="absolute bottom-1/4 right-1/4 h-64 w-64 animate-pulse rounded-full bg-purple-400/30 blur-3xl delay-700 dark:bg-purple-600/20" />
+      <div className="absolute bottom-1/4 right-1/4 h-64 w-64 animate-pulse rounded-full bg-gray-400/20 blur-3xl delay-700 dark:bg-gray-600/20" />
 
       {/* ========== 메인 콘텐츠 ========== */}
       <div className="relative z-10 w-full max-w-md animate-in fade-in slide-in-from-bottom-4 duration-700">
@@ -90,7 +90,7 @@ export default async function ResetPasswordPage({
         <Card className="border-white/20 bg-white/70 shadow-2xl backdrop-blur-xl dark:border-white/10 dark:bg-gray-900/70">
           <CardHeader className="space-y-3 text-center">
             {/* 아이콘 */}
-            <div className="mx-auto mb-2 flex h-16 w-16 items-center justify-center rounded-2xl bg-gradient-to-br from-indigo-500 to-purple-600 shadow-lg">
+            <div className="mx-auto mb-2 flex h-16 w-16 items-center justify-center rounded-2xl bg-gradient-to-br from-gray-800 to-black shadow-lg dark:from-white dark:to-gray-200">
               <span className="text-4xl">🔐</span>
             </div>
             {/* 페이지 제목 */}
@@ -132,7 +132,7 @@ export default async function ResetPasswordPage({
               {/* ========== 비밀번호 변경 버튼 ========== */}
               <Button
                 formAction={updatePassword}
-                className="h-11 w-full bg-gradient-to-r from-indigo-600 to-purple-600 font-semibold text-white shadow-lg transition-all hover:from-indigo-700 hover:to-purple-700 hover:shadow-xl"
+                className="h-11 w-full bg-gradient-to-r from-gray-900 to-black font-semibold text-white shadow-lg transition-all hover:from-black hover:to-gray-800 hover:shadow-xl dark:from-white dark:to-gray-100 dark:text-black dark:hover:from-gray-100 dark:hover:to-white"
               >
                 비밀번호 변경
               </Button>

app/signup/page.tsx

@@ -20,14 +20,14 @@ export default async function SignupPage({
   const { message } = await searchParams;
 
   return (
-    <div className="relative flex min-h-screen items-center justify-center overflow-hidden bg-gradient-to-br from-indigo-50 via-purple-50 to-pink-50 px-4 py-12 dark:from-gray-950 dark:via-indigo-950 dark:to-purple-950">
+    <div className="relative flex min-h-screen items-center justify-center overflow-hidden bg-gradient-to-br from-gray-50 via-white to-gray-100 px-4 py-12 dark:from-black dark:via-gray-950 dark:to-gray-900">
       {/* 배경 그라데이션 효과 */}
-      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top_right,_var(--tw-gradient-stops))] from-indigo-200/40 via-purple-200/20 to-transparent dark:from-indigo-900/30 dark:via-purple-900/20" />
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top_right,_var(--tw-gradient-stops))] from-gray-200/30 via-gray-100/15 to-transparent dark:from-gray-800/30 dark:via-gray-900/20" />
-      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_bottom_left,_var(--tw-gradient-stops))] from-pink-200/40 via-purple-200/20 to-transparent dark:from-pink-900/30 dark:via-purple-900/20" />
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_bottom_left,_var(--tw-gradient-stops))] from-gray-300/30 via-gray-200/15 to-transparent dark:from-gray-700/30 dark:via-gray-800/20" />
 
       {/* 애니메이션 블러 효과 */}
-      <div className="absolute left-1/4 top-1/4 h-64 w-64 animate-pulse rounded-full bg-indigo-400/30 blur-3xl dark:bg-indigo-600/20" />
+      <div className="absolute left-1/4 top-1/4 h-64 w-64 animate-pulse rounded-full bg-gray-300/20 blur-3xl dark:bg-gray-700/20" />
-      <div className="absolute bottom-1/4 right-1/4 h-64 w-64 animate-pulse rounded-full bg-purple-400/30 blur-3xl delay-700 dark:bg-purple-600/20" />
+      <div className="absolute bottom-1/4 right-1/4 h-64 w-64 animate-pulse rounded-full bg-gray-400/20 blur-3xl delay-700 dark:bg-gray-600/20" />
 
       <div className="relative z-10 w-full max-w-md animate-in fade-in slide-in-from-bottom-4 duration-700">
         {/* 메시지 알림 */}
@@ -35,7 +35,7 @@ export default async function SignupPage({
 
         <Card className="border-white/20 bg-white/70 shadow-2xl backdrop-blur-xl dark:border-white/10 dark:bg-gray-900/70">
           <CardHeader className="space-y-3 text-center">
-            <div className="mx-auto mb-2 flex h-16 w-16 items-center justify-center rounded-2xl bg-gradient-to-br from-indigo-500 to-purple-600 shadow-lg">
+            <div className="mx-auto mb-2 flex h-16 w-16 items-center justify-center rounded-2xl bg-gradient-to-br from-gray-800 to-black shadow-lg dark:from-white dark:to-gray-200">
               <span className="text-4xl">🚀</span>
             </div>
             <CardTitle className="text-3xl font-bold tracking-tight">
@@ -91,7 +91,7 @@ export default async function SignupPage({
               <Button
                 formAction={signup}
                 type="submit"
-                className="h-11 w-full bg-gradient-to-r from-indigo-600 to-purple-600 font-semibold shadow-lg transition-all duration-200 hover:from-indigo-700 hover:to-purple-700 hover:shadow-xl"
+                className="h-11 w-full bg-gradient-to-r from-gray-900 to-black font-semibold shadow-lg transition-all duration-200 hover:from-black hover:to-gray-800 hover:shadow-xl dark:from-white dark:to-gray-100 dark:text-black dark:hover:from-gray-100 dark:hover:to-white"
                 size="lg"
               >
                 회원가입 완료
@@ -102,7 +102,7 @@ export default async function SignupPage({
                 이미 계정이 있으신가요?{" "}
                 <Link
                   href="/login"
-                  className="font-semibold text-indigo-600 transition-colors hover:text-indigo-500 dark:text-indigo-400 dark:hover:text-indigo-300"
+                  className="font-semibold text-gray-900 transition-colors hover:text-black dark:text-gray-100 dark:hover:text-white"
                 >
                   로그인 하러 가기
                 </Link>

features/auth/actions.ts

@@ -385,3 +385,94 @@ export async function updatePassword(formData: FormData) {
     `/login?message=${encodeURIComponent(AUTH_ERROR_MESSAGES.PASSWORD_RESET_SUCCESS)}`,
   );
 }
+
+// ========================================
+// 소셜 로그인 (OAuth)
+// ========================================
+
+/**
+ * [OAuth 로그인 헬퍼 함수]
+ *
+ * Google, Kakao 등의 소셜 로그인 제공자를 통해 인증을 수행합니다.
+ *
+ * PKCE (Proof Key for Code Exchange) 플로우:
+ * 1. 이 함수가 signInWithOAuth를 호출하면 Supabase가 OAuth URL 반환
+ * 2. 사용자를 해당 OAuth 제공자(Google/Kakao) 로그인 페이지로 리다이렉트
+ * 3. 사용자가 로그인 및 권한 동의
+ * 4. OAuth 제공자가 /auth/callback?code=xxx로 리다이렉트
+ * 5. callback 라우트에서 code를 세션으로 교환 (exchangeCodeForSession)
+ * 6. 메인 페이지로 최종 리다이렉트
+ *
+ * 자동 회원가입:
+ * - 첫 로그인 시 auth.users 테이블에 자동으로 사용자 생성
+ * - 이메일, provider, metadata(프로필 사진, 이름 등) 저장
+ * - 기존 사용자는 그냥 로그인
+ *
+ * @param provider - OAuth 제공자 ('google' | 'kakao')
+ */
+async function signInWithProvider(provider: "google" | "kakao") {
+  const supabase = await createClient();
+
+  // 1. OAuth 인증 시작
+  const { data, error } = await supabase.auth.signInWithOAuth({
+    provider,
+    options: {
+      // PKCE 플로우를 위한 콜백 URL
+      // OAuth 제공자 인증 후 이 URL로 code와 함께 리다이렉트됨
+      redirectTo: `${process.env.NEXT_PUBLIC_BASE_URL || "http://localhost:3001"}/auth/callback`,
+    },
+  });
+
+  // 2. 에러 처리
+  if (error) {
+    console.error(`[${provider} OAuth] 로그인 실패:`, error.message);
+    return redirect(
+      `/login?message=${encodeURIComponent(`${provider} 로그인에 실패했습니다. 다시 시도해 주세요.`)}`,
+    );
+  }
+
+  // 3. OAuth 제공자 로그인 페이지로 리다이렉트
+  // data.url은 Google/Kakao의 인증 페이지 URL
+  if (data.url) {
+    redirect(data.url);
+  }
+
+  // 4. URL이 없는 경우 (예상치 못한 상황)
+  redirect(
+    `/login?message=${encodeURIComponent("로그인 처리 중 오류가 발생했습니다.")}`,
+  );
+}
+
+/**
+ * [Google 로그인 액션]
+ *
+ * Google 계정으로 로그인합니다.
+ * "Google로 로그인" 버튼에서 호출됩니다.
+ *
+ * 처리 과정:
+ * 1. signInWithOAuth 호출하여 Google OAuth URL 받기
+ * 2. Google 로그인 페이지로 리다이렉트
+ * 3. (사용자가 Google에서 로그인 및 권한 동의)
+ * 4. /auth/callback으로 돌아와서 세션 생성
+ * 5. 메인 페이지로 이동
+ */
+export async function signInWithGoogle() {
+  return signInWithProvider("google");
+}
+
+/**
+ * [Kakao 로그인 액션]
+ *
+ * Kakao 계정으로 로그인합니다.
+ * "Kakao로 로그인" 버튼에서 호출됩니다.
+ *
+ * 처리 과정:
+ * 1. signInWithOAuth 호출하여 Kakao OAuth URL 받기
+ * 2. Kakao 로그인 페이지로 리다이렉트
+ * 3. (사용자가 Kakao에서 로그인 및 권한 동의)
+ * 4. /auth/callback으로 돌아와서 세션 생성
+ * 5. 메인 페이지로 이동
+ */
+export async function signInWithKakao() {
+  return signInWithProvider("kakao");
+}