- Remove auto-account-creation fallback on auth/user-not-found during
  signIn — sign-in should only sign in, not silently create accounts
- Add missing Firebase error mappings: email-already-exists,
  invalid-login-credentials, operation-not-allowed, internal-error
- Move pageState.update before onNavigate lifecycle hook so route
  components see the updated state when onNavigate fires
- Make onAuthStateChanged handler async/await for proper error propagation