refactor(auth): inline NextAuth v4 config and simplify AuthPlatform API

- Inline NextAuth v4 handler directly in `[...nextauth]/route.ts`, removing
  the `@/lib/auth/nextauth` abstraction layer
- Add Google/Facebook providers with JWT/session callbacks that expose
  `idToken` and `accessToken` on the session for backend exchange
- Refactor `AuthPlatform` from constructor-based to static methods
  (`googleSignIn()` / `facebookSignIn()`)
- Update `auth-machine.ts` and layout comment reference to match new structure
- Align with original Dart `nextauth-helpers.{googleLogin, facebookLogin}()`
  naming and keep persistence concerns (unstorage, backend, custom cookies)
  out of scope for this iteration

src/stores/auth/auth-machine.ts

@@ -9,9 +9,8 @@
  * `next-auth/react.signIn`（OAuth 跳转）。
  */
 import { setup, fromPromise, assign } from "xstate";
-import { signIn } from "next-auth/react";
 
-import type { AuthProvider } from "@/lib/auth/auth_platform";
+import { AuthPlatform, type AuthProvider } from "@/lib/auth/auth_platform";
 import type { LoginStatus } from "@/models/auth/login-status";
 import { authRepository } from "@/data/repositories/auth_repository";
 import type { IAuthRepository } from "@/data/repositories/interfaces";
@@ -72,11 +71,15 @@ const emailRegisterThenLoginActor = fromPromise<
   return "email" as LoginStatus;
 });
 
-// OAuth 登录 actor：调 next-auth/react 的 signIn(provider) 触发 OAuth 跳转。
+// OAuth 登录 actor：调 AuthPlatform 触发 OAuth 跳转。
 // 成功路径下 NextAuth 会重定向离开本应用，actor 通常不会 resolve；
 // onDone 仍保留以处理 SDK 同步返回（极少见）。
 const oauthSignInActor = fromPromise<void, AuthProvider>(async ({ input }) => {
-  await signIn(input);
+  if (input === "google") {
+    await AuthPlatform.googleSignIn();
+  } else {
+    await AuthPlatform.facebookSignIn();
+  }
 });
 
 // ========== OAuth token → 后端业务 token sync actors ==========