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_dev.dart

@@ -0,0 +1,14 @@
+import 'firebase/firebase_options_dev.dart';
+import 'main.dart';
+
+/// DEV flavor entrypoint.
+///
+/// Run/build with the matching flavor + env file:
+///   flutter run --flavor dev -t lib/main_dev.dart \
+///     --dart-define-from-file=env/dev.json
+void main() {
+  bootstrap(
+    firebaseOptions: DevFirebaseOptions.currentPlatform,
+    flavor: 'dev',
+  );
+}