- 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
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