feat(build): add dev/staging/prod flavors for client_app + mitra_app

Android product flavors (.dev/.staging suffixes, prod clean) + per-flavor
Dart entrypoints, dart-define env files, and per-flavor Firebase config for
both platforms across 3 projects (halobestie-clone-dev / my-bestie-876ec /
my-bestie-production).

- Android: flavorDimensions("env") + productFlavors; @string/app_name label;
  per-flavor src/<flavor>/google-services.json (clients verified to match each
  applicationId).
- iOS: customer app re-based to the EXISTING App Store identity
  com.asc.hallobestie (dev/staging suffix it; ships as an update to the live
  app). mitra is a new app (com.mybestie.mitra). Per-flavor plists staged in
  ios/config/<flavor>/; Xcode scheme wiring deferred (Mac follow-up).
- firebase_options_{dev,staging,prod}.dart filled with real android + iOS
  values (regenerated from the native config files).
- BUILD_FLAVORS.md per app documents flavor table, build commands, iOS
  identity decision, and the remaining iOS Xcode steps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client_app/lib/main.dart

@@ -13,10 +13,23 @@ import 'core/chat/chat_notifier.dart';
 import 'core/notifications/notification_service.dart';
 import 'core/pairing/pairing_notifier.dart';
 import 'core/theme/halo_theme.dart';
-import 'firebase_options.dart';
+import 'firebase/firebase_options_dev.dart';
 import 'router.dart';
 
-void main() async {
+/// Shared app bootstrap, parameterised per build flavor.
+///
+/// The flavor entrypoints (`main_dev.dart`, `main_staging.dart`,
+/// `main_prod.dart`) each call this with their environment's
+/// [FirebaseOptions] and a [flavor] tag. The bare [main] below delegates to
+/// dev so a plain `flutter run` (no `-t`) still launches the dev environment.
+///
+/// `flavor` is currently informational (kept on hand for future flavor-gated
+/// behaviour / analytics tagging); the API base URL is supplied separately via
+/// `--dart-define-from-file=env/<flavor>.json` (see BUILD_FLAVORS.md).
+Future<void> bootstrap({
+  required FirebaseOptions firebaseOptions,
+  required String flavor,
+}) async {
   WidgetsFlutterBinding.ensureInitialized();
 
   // Pre-warm flutter_secure_storage. The first call triggers AndroidX
@@ -26,7 +39,7 @@ void main() async {
   // splash instead of paying it on the user's first interaction.
   unawaited(TokenStorage().readRefreshToken());
 
-  await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);
+  await Firebase.initializeApp(options: firebaseOptions);
 
   final messaging = FirebaseMessaging.instance;
   await messaging.requestPermission();
@@ -34,6 +47,16 @@ void main() async {
   runApp(const ProviderScope(child: App()));
 }
 
+void main() async {
+  // Bare `flutter run` (no `-t lib/main_<flavor>.dart`) defaults to dev so
+  // local development works out of the box. Build-flavor APKs use the
+  // flavor-specific entrypoints instead.
+  await bootstrap(
+    firebaseOptions: DevFirebaseOptions.currentPlatform,
+    flavor: 'dev',
+  );
+}
+
 class App extends ConsumerStatefulWidget {
   const App({super.key});