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/auth/components/auth-facebook-panel.tsx

@@ -7,13 +7,13 @@
  * 视觉规格（与 Dart 对齐）：
  *   - Spacer → logo（120h）→ Spacer → Facebook 主按钮 → 链接 → Spacer → 法律
  *   - Facebook 主按钮：46px 白胶囊 + Facebook "f" 图标（#1877f2）+ "Login with Facebook"
- *   - 社交登录走 NextAuth：`facebookLogin()` / `googleLogin()`
+ *   - 社交登录走 NextAuth：`new FacebookLogin().signIn()` / `new GoogleLogin().signIn()`
  *   - "Other Sign-in Options" 链接打开底部弹层
  *   - 弹层 Email 按钮 → 触发 `onSwitchToEmail`（父级派发 `AuthPanelModeChanged`）
  */
 import { useState } from "react";
 
-import { facebookLogin, googleLogin } from "@/lib/auth/nextauth";
+import { FacebookLogin, GoogleLogin } from "@/lib/auth/nextauth";
 
 import { AuthErrorMessage } from "./auth-error-message";
 import { AuthLegalText } from "./auth-legal-text";
@@ -32,17 +32,25 @@ export function AuthFacebookPanel({ onSwitchToEmail }: AuthFacebookPanelProps) {
   const handleFacebook = async () => {
     setBusy("facebook");
     setError(null);
-    const r = await facebookLogin();
-    setBusy(null);
-    if (!r.success) setError(r.error?.message ?? "Facebook login failed");
+    try {
+      await new FacebookLogin().signIn();
+    } catch (e) {
+      setError(e instanceof Error ? e.message : "Facebook login failed");
+    } finally {
+      setBusy(null);
+    }
   };
 
   const handleGoogle = async () => {
     setBusy("google");
     setError(null);
-    const r = await googleLogin();
-    setBusy(null);
-    if (!r.success) setError(r.error?.message ?? "Google login failed");
+    try {
+      await new GoogleLogin().signIn();
+    } catch (e) {
+      setError(e instanceof Error ? e.message : "Google login failed");
+    } finally {
+      setBusy(null);
+    }
   };
 
   return (