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/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_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_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-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 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 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-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>