Implement Google->Firebase->Backend auth flow

- Add authGoogleNew GraphQL mutation and token exchange in AuthServiceImpl
- Add ApiTokenStore and SharedPreferencesApiTokenStore for API token persistence
- Add ApolloAuthInterceptor to inject Bearer token on GraphQL requests
- Introduce FirebaseSessionManager for testable Firebase auth orchestration
- Update LoginViewModel to surface backend auth errors
- Add unit tests for Firebase failure, backend failure, and auth state

Made-with: Cursor

app/src/main/kotlin/org/db3/airmq/features/login/LoginViewModel.kt

@@ -78,7 +78,12 @@ class LoginViewModel @Inject constructor(
             if (signInResult.isSuccess) {
                 _actions.tryEmit(Action.OpenManage)
             } else {
-                _actions.tryEmit(Action.ShowMessage(appContext.getString(R.string.toast_oauth_failed)))
+                val message = signInResult.exceptionOrNull()?.message
+                _actions.tryEmit(
+                    Action.ShowMessage(
+                        message ?: appContext.getString(R.string.toast_oauth_failed)
+                    )
+                )
             }
         }
     }