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 };

src/app/api/auth/marker/route.ts

@@ -1,51 +0,0 @@
-/**
- * 路由信号 cookie 管理端点
- *
- * 用途：写/清 `cozsweet_authed` httpOnly cookie，作为 `src/router/proxy.ts`
- * 的"用户已登录"信号源。
- *
- * 与 `next-auth.session-token` 的区别：
- *   - NextAuth cookie 仅社交登录设置，邮箱登录不写入
- *   - 本端点由所有登录出口（社交 / 邮箱）统一调用，proxy 只需读这一个 cookie
- *
- * 调用方：
- *   - 社交登录出口（nextauth-helpers.persistBackendSession，**server 端**直接用
- *     `next/headers.cookies()` 写，不走本端点）
- *   - 邮箱登录出口（auth-machine.emailLoginActor / emailRegisterThenLoginActor，
- *     **client 端** fetch POST）
- *   - 登出（nextauth-helpers.logout，client 端 fetch DELETE）
- *
- * 安全：cookie 设为 `HttpOnly` + `SameSite=Lax`，prod 额外 `Secure`。
- */
-
-import { NextResponse } from "next/server";
-
-const COOKIE_NAME = "cozsweet_authed";
-const ONE_WEEK = 60 * 60 * 24 * 7;
-
-/** 设置 httpOnly 路由信号 cookie（任何登录成功后调用） */
-export async function POST() {
-  const res = NextResponse.json({ ok: true });
-  res.cookies.set({
-    name: COOKIE_NAME,
-    value: "1",
-    httpOnly: true,
-    secure: process.env.NODE_ENV === "production",
-    sameSite: "lax",
-    path: "/",
-    maxAge: ONE_WEEK,
-  });
-  return res;
-}
-
-/** 清除路由信号 cookie（登出时调用） */
-export async function DELETE() {
-  const res = NextResponse.json({ ok: true });
-  res.cookies.set({
-    name: COOKIE_NAME,
-    value: "",
-    path: "/",
-    maxAge: 0,
-  });
-  return res;
-}

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 (

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);
     }
   };