refactor(auth): switch to class-based social login and v4 route handler

Migrate from function-based social login helpers (`facebookLogin`, `googleLogin`)
to class-based services (`new FacebookLogin().signIn()`, `new GoogleLogin().signIn()`),
updating call sites in `AuthFacebookPanel` and `SplashButton` with try/catch error
handling. The NextAuth route handler is also refactored to the v4 pattern, importing
pre-built `GET` / `POST` exports from `@/lib/auth/nextauth` instead of constructing
the handler inline with `NextAuth(authOptions)`.

src/app/api/auth/[...nextauth]/route.ts

@@ -1,8 +1,7 @@
 /**
- * NextAuth App Router 入口
+ * NextAuth App Router 入口（v4 模式）
  *
- * 路径：/api/auth/[...nextauth]
- * 自动处理 NextAuth 所有内置路由：
+ * v4 用单个 handler 函数，需手动 re-export 为 `GET` / `POST`：
  *   /api/auth/signin/[provider]
  *   /api/auth/callback/[provider]
  *   /api/auth/signout
@@ -10,10 +9,6 @@
  *   /api/auth/csrf
  *   /api/auth/providers
  */
-import NextAuth from "next-auth";
+import { GET, POST } from "@/lib/auth/nextauth";
 
-import { authOptions } from "@/lib/auth/config";
-
-const handler = NextAuth(authOptions);
-
-export { handler as GET, handler as POST };
+export { GET, POST };