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

@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>CLIENT_ID</key>
+	<string>953866659887-bsb3c2a6ir10u47q8vcacre2tmnk59jb.apps.googleusercontent.com</string>
+	<key>REVERSED_CLIENT_ID</key>
+	<string>com.googleusercontent.apps.953866659887-bsb3c2a6ir10u47q8vcacre2tmnk59jb</string>
+	<key>ANDROID_CLIENT_ID</key>
+	<string>953866659887-5a62u0tdce92i0gmfo0gf3dt0dnlre42.apps.googleusercontent.com</string>
+	<key>API_KEY</key>
+	<string>AIzaSyA8guPSD87eDLeCsH6jVd1n2_SI4_MaGNE</string>
+	<key>GCM_SENDER_ID</key>
+	<string>953866659887</string>
+	<key>PLIST_VERSION</key>
+	<string>1</string>
+	<key>BUNDLE_ID</key>
+	<string>com.asc.hallobestie</string>
+	<key>PROJECT_ID</key>
+	<string>my-bestie-production</string>
+	<key>STORAGE_BUCKET</key>
+	<string>my-bestie-production.firebasestorage.app</string>
+	<key>IS_ADS_ENABLED</key>
+	<false></false>
+	<key>IS_ANALYTICS_ENABLED</key>
+	<false></false>
+	<key>IS_APPINVITE_ENABLED</key>
+	<true></true>
+	<key>IS_GCM_ENABLED</key>
+	<true></true>
+	<key>IS_SIGNIN_ENABLED</key>
+	<true></true>
+	<key>GOOGLE_APP_ID</key>
+	<string>1:953866659887:ios:159fd11b1d2f3633183eda</string>
+</dict>
+</plist>