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/android/app/build.gradle.kts

@@ -24,7 +24,10 @@ android {
     }
 
     defaultConfig {
-        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
+        // Base application ID. Per-flavor suffixes below produce the final IDs:
+        //   dev     -> com.mybestie.dev
+        //   staging -> com.mybestie.staging
+        //   prod    -> com.mybestie
         applicationId = "com.mybestie"
         // You can update the following values to match your application needs.
         // For more information, see: https://flutter.dev/to/review-gradle-config.
@@ -34,6 +37,34 @@ android {
         versionName = flutter.versionName
     }
 
+    // Build flavors for the three environments. Each flavor:
+    //   - sets its final applicationId (via suffix, except prod)
+    //   - injects an `app_name` string resource consumed by
+    //     AndroidManifest.xml's android:label="@string/app_name"
+    //   - selects its own Firebase config via the matching source set
+    //     (android/app/src/<flavor>/google-services.json)
+    // NOTE: once these flavors exist, a bare `flutter build apk` (no --flavor)
+    // fails. All build/install/run commands MUST pass --flavor. See
+    // BUILD_FLAVORS.md.
+    flavorDimensions += "env"
+    productFlavors {
+        create("dev") {
+            dimension = "env"
+            applicationIdSuffix = ".dev"
+            resValue("string", "app_name", "HaloBestie Dev")
+        }
+        create("staging") {
+            dimension = "env"
+            applicationIdSuffix = ".staging"
+            resValue("string", "app_name", "HaloBestie Staging")
+        }
+        create("prod") {
+            dimension = "env"
+            // No applicationIdSuffix -> final applicationId stays "com.mybestie".
+            resValue("string", "app_name", "HaloBestie")
+        }
+    }
+
     buildTypes {
         release {
             // TODO: Add your own signing config for the release build.