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/src/dev/google-services.json

@@ -0,0 +1,105 @@
+{
+  "project_info": {
+    "project_number": "1068156046511",
+    "project_id": "halobestie-clone-dev",
+    "storage_bucket": "halobestie-clone-dev.firebasestorage.app"
+  },
+  "client": [
+    {
+      "client_info": {
+        "mobilesdk_app_id": "1:1068156046511:android:ba6e699216de1c50b8185a",
+        "android_client_info": {
+          "package_name": "com.halobestie.client.client_app"
+        }
+      },
+      "oauth_client": [],
+      "api_key": [
+        {
+          "current_key": "AIzaSyDFWlLSWytqwI7LSdUbVrO7J5De9L2LV2U"
+        }
+      ],
+      "services": {
+        "appinvite_service": {
+          "other_platform_oauth_client": []
+        }
+      }
+    },
+    {
+      "client_info": {
+        "mobilesdk_app_id": "1:1068156046511:android:f30784f6b0423131b8185a",
+        "android_client_info": {
+          "package_name": "com.halobestie.mitra"
+        }
+      },
+      "oauth_client": [],
+      "api_key": [
+        {
+          "current_key": "AIzaSyDFWlLSWytqwI7LSdUbVrO7J5De9L2LV2U"
+        }
+      ],
+      "services": {
+        "appinvite_service": {
+          "other_platform_oauth_client": []
+        }
+      }
+    },
+    {
+      "client_info": {
+        "mobilesdk_app_id": "1:1068156046511:android:4f8fe9a3c7c14c57b8185a",
+        "android_client_info": {
+          "package_name": "com.mybestie"
+        }
+      },
+      "oauth_client": [],
+      "api_key": [
+        {
+          "current_key": "AIzaSyDFWlLSWytqwI7LSdUbVrO7J5De9L2LV2U"
+        }
+      ],
+      "services": {
+        "appinvite_service": {
+          "other_platform_oauth_client": []
+        }
+      }
+    },
+    {
+      "client_info": {
+        "mobilesdk_app_id": "1:1068156046511:android:1f589ed358ccdad0b8185a",
+        "android_client_info": {
+          "package_name": "com.mybestie.dev"
+        }
+      },
+      "oauth_client": [],
+      "api_key": [
+        {
+          "current_key": "AIzaSyDFWlLSWytqwI7LSdUbVrO7J5De9L2LV2U"
+        }
+      ],
+      "services": {
+        "appinvite_service": {
+          "other_platform_oauth_client": []
+        }
+      }
+    },
+    {
+      "client_info": {
+        "mobilesdk_app_id": "1:1068156046511:android:8a873c8b7e64410ab8185a",
+        "android_client_info": {
+          "package_name": "com.mybestie.mitra"
+        }
+      },
+      "oauth_client": [],
+      "api_key": [
+        {
+          "current_key": "AIzaSyDFWlLSWytqwI7LSdUbVrO7J5De9L2LV2U"
+        }
+      ],
+      "services": {
+        "appinvite_service": {
+          "other_platform_oauth_client": []
+        }
+      }
+    }
+  ],
+  "configuration_version": "1"
+}