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;
-}