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

sdk/build.gradle.kts

@@ -62,4 +62,6 @@ dependencies {
     ksp(libs.hilt.compiler)
 
     testImplementation(libs.junit)
+    testImplementation(libs.kotlinx.coroutines.test)
+    testImplementation(libs.mockk.jvm)
 }