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/splash/components/splash-button.tsx

@@ -11,7 +11,7 @@
  *   - 消费 `useChatDispatch` / `useUserDispatch`（登录成功后初始化）
  *   - 自管路由跳转（已登录 / 登录成功 → /chat）
  *   - Skip 用 `<Link>` 保留 SPA 体验
- *   - 社交登录按钮直接调 `nextauth.facebookLogin()`（NextAuth 流程）
+ *   - 社交登录按钮走 `new FacebookLogin().signIn()`（NextAuth 流程）
  */
 import Link from "next/link";
 import { useRouter } from "next/navigation";
@@ -20,7 +20,7 @@ import { useEffect, useRef } from "react";
 import { useAuthState } from "@/stores/auth/auth-context";
 import { useChatDispatch } from "@/stores/chat/chat-context";
 import { useUserDispatch } from "@/stores/user/user-context";
-import { facebookLogin } from "@/lib/auth/nextauth";
+import { FacebookLogin } from "@/lib/auth/nextauth";
 import { ROUTES } from "@/router/routes";
 import { LoadingIndicator } from "@/app/_components/core/loading-indicator";
 import styles from "./splash-button.module.css";
@@ -57,9 +57,10 @@ export function SplashButton({ skipHref = ROUTES.chat }: SplashButtonProps) {
   }, [state.isSuccess, chatDispatch, userDispatch, router]);
 
   const handleFacebookLogin = async () => {
-    const r = await facebookLogin();
-    if (!r.success) {
-      console.error("[splash-button] Facebook login failed", r.error);
+    try {
+      await new FacebookLogin().signIn();
+    } catch (e) {
+      console.error("[splash-button] Facebook login failed", e);
     }
   };