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>

mitra_app/lib/bootstrap.dart

@@ -0,0 +1,125 @@
+import 'package:firebase_core/firebase_core.dart';
+import 'package:firebase_messaging/firebase_messaging.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'core/api/api_client_provider.dart';
+import 'core/auth/auth_notifier.dart';
+import 'core/chat/mitra_chat_notifier.dart';
+import 'core/status/status_notifier.dart';
+import 'core/chat/chat_request_notifier.dart';
+import 'core/chat/widgets/chat_request_overlay.dart';
+import 'core/notifications/notification_service.dart';
+import 'core/theme/halo_theme.dart';
+import 'router.dart';
+
+/// Shared app bootstrap used by every flavor entrypoint
+/// (main_dev / main_staging / main_prod). Each entrypoint passes the
+/// flavor's own [FirebaseOptions] and a [flavor] tag.
+Future<void> bootstrap({
+  required FirebaseOptions firebaseOptions,
+  required String flavor,
+}) async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await Firebase.initializeApp(options: firebaseOptions);
+
+  final messaging = FirebaseMessaging.instance;
+  await messaging.requestPermission();
+
+  runApp(const ProviderScope(child: App()));
+}
+
+class App extends ConsumerStatefulWidget {
+  const App({super.key});
+
+  @override
+  ConsumerState<App> createState() => _AppState();
+}
+
+class _AppState extends ConsumerState<App> with WidgetsBindingObserver {
+  bool _fcmRegistered = false;
+  // Session the chat WS was on at the moment we backgrounded. Restored on
+  // resume so a backgrounded mitra reconnects to the same chat once they
+  // foreground the app. Mirrors the customer-app fix (main.dart on the
+  // client side) — backend's sendMessage checks recipient WS readyState
+  // before falling back to FCM, so leaving the WS open while paused makes
+  // FCM never fire and the mitra misses customer messages in background.
+  String? _pausedChatSessionId;
+
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addObserver(this);
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.paused || state == AppLifecycleState.detached) {
+      ref.read(onlineStatusProvider.notifier).onAppPaused();
+      // Close the chat WS so backend `sendMessage` falls back to FCM when
+      // the customer sends a message. Stash the active session_id so we
+      // can rejoin it on resume.
+      final chatNotifier = ref.read(mitraChatProvider.notifier);
+      final sid = chatNotifier.connectedSessionId;
+      if (sid != null) {
+        _pausedChatSessionId = sid;
+        chatNotifier.disconnect();
+      }
+    } else if (state == AppLifecycleState.resumed) {
+      ref.read(onlineStatusProvider.notifier).onAppResumed();
+      // Reconnect to the chat we backgrounded out of, if any.
+      final saved = _pausedChatSessionId;
+      _pausedChatSessionId = null;
+      if (saved != null) {
+        // ignore: discarded_futures
+        ref.read(mitraChatProvider.notifier).connect(saved);
+      }
+    }
+  }
+
+  void _registerFcmToken() {
+    if (_fcmRegistered) return;
+    _fcmRegistered = true;
+    Future(() async {
+      try {
+        final token = await FirebaseMessaging.instance.getToken();
+        if (token != null) {
+          await ref.read(apiClientProvider).post('/api/shared/device-token', data: {'token': token});
+        }
+      } catch (_) {
+        _fcmRegistered = false;
+      }
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    // Listen for auth changes to load status and register FCM
+    ref.listen(mitraAuthProvider, (prev, next) {
+      final data = next.valueOrNull;
+      if (data is MitraAuthAuthenticatedData) {
+        ref.read(onlineStatusProvider.notifier).load();
+        _registerFcmToken();
+      }
+    });
+
+    final router = ref.watch(routerProvider);
+    NotificationService.initialize(router);
+    NotificationService.onChatRequestTapped = (sessionId) {
+      ref.read(chatRequestProvider.notifier).setIncomingFromNotification(sessionId);
+    };
+
+    return ChatRequestOverlay(
+      child: MaterialApp.router(
+        title: 'Halo Bestie Mitra',
+        theme: haloThemeData(),
+        routerConfig: router,
+      ),
+    );
+  }
+}