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/prod/google-services.json

@@ -0,0 +1,70 @@
+{
+  "project_info": {
+    "project_number": "953866659887",
+    "project_id": "my-bestie-production",
+    "storage_bucket": "my-bestie-production.firebasestorage.app"
+  },
+  "client": [
+    {
+      "client_info": {
+        "mobilesdk_app_id": "1:953866659887:android:55dfbf97ac7c26e7183eda",
+        "android_client_info": {
+          "package_name": "com.mybestie"
+        }
+      },
+      "oauth_client": [
+        {
+          "client_id": "953866659887-5a62u0tdce92i0gmfo0gf3dt0dnlre42.apps.googleusercontent.com",
+          "client_type": 1,
+          "android_info": {
+            "package_name": "com.mybestie",
+            "certificate_hash": "3c43db3c9ac7f6d7e2fa03b8dbcaf7e5d12c97f3"
+          }
+        },
+        {
+          "client_id": "953866659887-kebg3eijcomtv97q6v03fm8i30kj7r9r.apps.googleusercontent.com",
+          "client_type": 1,
+          "android_info": {
+            "package_name": "com.mybestie",
+            "certificate_hash": "7119b6cf7091074759450c899191905a5a4d0369"
+          }
+        },
+        {
+          "client_id": "953866659887-vvur2mnmbu8ljmnmmg01hqsrj0ocssu9.apps.googleusercontent.com",
+          "client_type": 1,
+          "android_info": {
+            "package_name": "com.mybestie",
+            "certificate_hash": "6e23f0164af04ddf200f769c460caac3ee2b91ac"
+          }
+        },
+        {
+          "client_id": "953866659887-9ro36er68qupv00rgokjignnu2hs85v8.apps.googleusercontent.com",
+          "client_type": 3
+        }
+      ],
+      "api_key": [
+        {
+          "current_key": "AIzaSyAxAp8hcXO-P7HwwVsS3vFe0OX5ZkIyyWI"
+        }
+      ],
+      "services": {
+        "appinvite_service": {
+          "other_platform_oauth_client": [
+            {
+              "client_id": "953866659887-9ro36er68qupv00rgokjignnu2hs85v8.apps.googleusercontent.com",
+              "client_type": 3
+            },
+            {
+              "client_id": "953866659887-bsb3c2a6ir10u47q8vcacre2tmnk59jb.apps.googleusercontent.com",
+              "client_type": 2,
+              "ios_info": {
+                "bundle_id": "com.asc.hallobestie"
+              }
+            }
+          ]
+        }
+      }
+    }
+  ],
+  "configuration_version": "1"
+}