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/ios/config/prod/GoogleService-Info.plist.README

@@ -0,0 +1,16 @@
+Place the PROD GoogleService-Info.plist here, named exactly:
+
+    GoogleService-Info.plist
+
+Register this iOS app first:
+  • Firebase project: the PRODUCTION project
+  • iOS bundle ID:    com.mybestie.mitra   (brand-new app — mitra has no legacy
+                       App Store identity, unlike the customer app)
+Then download its GoogleService-Info.plist into this folder (delete this .README).
+
+Also paste its values into lib/firebase/firebase_options_prod.dart
+(currently all PLACEHOLDER).
+
+How it gets used: a build-phase "Run Script" (added in Xcode — Mac follow-up)
+copies the per-flavor file into ios/Runner/GoogleService-Info.plist at build
+time. See ../README.md and BUILD_FLAVORS.md.