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/types/next-auth.d.ts

@@ -3,7 +3,7 @@ import "next-auth";
 /**
  * 扩展 next-auth v4 的 Session 类型
  *
- * `src/lib/auth/nextauth.ts` 的 jwt/session callbacks 把
+ * `src/app/api/auth/[...nextauth]/route.ts` 的 jwt/session callbacks 把
  *   - OAuth provider 名（"google" | "facebook"）
  *   - Google id_token
  *   - Facebook access_token