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/firebase/firebase_options_prod.dart

@@ -0,0 +1,61 @@
+// File generated by FlutterFire CLI (regenerated from the registered
+// prod Firebase apps — project my-bestie-production).
+// ignore_for_file: type=lint
+import 'package:firebase_core/firebase_core.dart' show FirebaseOptions;
+import 'package:flutter/foundation.dart'
+    show defaultTargetPlatform, kIsWeb, TargetPlatform;
+
+/// [FirebaseOptions] for the PROD environment (project my-bestie-production).
+class ProdFirebaseOptions {
+  static FirebaseOptions get currentPlatform {
+    if (kIsWeb) {
+        throw UnsupportedError(
+          'DefaultFirebaseOptions have not been configured for web - '
+          'you can reconfigure this by running the FlutterFire CLI again.',
+        );
+    }
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.android:
+        return android;
+      case TargetPlatform.iOS:
+        return ios;
+      case TargetPlatform.macOS:
+        throw UnsupportedError(
+          'DefaultFirebaseOptions have not been configured for macos - '
+          'you can reconfigure this by running the FlutterFire CLI again.',
+        );
+      case TargetPlatform.windows:
+        throw UnsupportedError(
+          'DefaultFirebaseOptions have not been configured for windows - '
+          'you can reconfigure this by running the FlutterFire CLI again.',
+        );
+      case TargetPlatform.linux:
+        throw UnsupportedError(
+          'DefaultFirebaseOptions have not been configured for linux - '
+          'you can reconfigure this by running the FlutterFire CLI again.',
+        );
+      default:
+        throw UnsupportedError(
+          'DefaultFirebaseOptions are not supported for this platform.',
+        );
+    }
+  }
+
+  static const FirebaseOptions android = FirebaseOptions(
+    apiKey: 'AIzaSyAxAp8hcXO-P7HwwVsS3vFe0OX5ZkIyyWI',
+    appId: '1:953866659887:android:55dfbf97ac7c26e7183eda',
+    messagingSenderId: '953866659887',
+    projectId: 'my-bestie-production',
+    storageBucket: 'my-bestie-production.firebasestorage.app',
+  );
+
+  static const FirebaseOptions ios = FirebaseOptions(
+    apiKey: 'AIzaSyA8guPSD87eDLeCsH6jVd1n2_SI4_MaGNE',
+    appId: '1:953866659887:ios:159fd11b1d2f3633183eda',
+    messagingSenderId: '953866659887',
+    projectId: 'my-bestie-production',
+    storageBucket: 'my-bestie-production.firebasestorage.app',
+    iosClientId: '953866659887-bsb3c2a6ir10u47q8vcacre2tmnk59jb.apps.googleusercontent.com',
+    iosBundleId: 'com.asc.hallobestie',
+  );
+}