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/README.md

@@ -0,0 +1,34 @@
+# iOS per-flavor Firebase config — mitra_app
+
+iOS has **no** automatic per-flavor resolution (unlike Android's
+`android/app/src/<flavor>/google-services.json`). Stage the three
+`GoogleService-Info.plist` files here, one per flavor:
+
+```
+ios/config/
+  dev/GoogleService-Info.plist        → bundle com.mybestie.mitra.dev      (dev project)
+  staging/GoogleService-Info.plist    → bundle com.mybestie.mitra.staging  (staging/nonprod)
+  prod/GoogleService-Info.plist       → bundle com.mybestie.mitra          (prod project)
+```
+
+> The mitra app is a **brand-new app** on both platforms — no legacy App Store
+> identity (unlike the customer app, which inherits `com.asc.hallobestie`).
+
+## Wiring (Xcode — Mac follow-up, not done yet)
+
+The active plist Xcode bundles is `ios/Runner/GoogleService-Info.plist`. To make
+it per-flavor, add a **Run Script** build phase to the Runner target, placed
+**before** "Compile Sources":
+
+```bash
+cp "${PROJECT_DIR}/config/${FLAVOR}/GoogleService-Info.plist" \
+   "${PROJECT_DIR}/Runner/GoogleService-Info.plist"
+```
+
+`${FLAVOR}` is a per-scheme/configuration build setting you define when creating
+the dev/staging/prod schemes (e.g. `FLAVOR = dev` in the dev configuration).
+
+### Single-env shortcut
+If you only need one env working (e.g. dev) before the full scheme setup, just
+drop that env's plist directly at `ios/Runner/GoogleService-Info.plist` — no
+script needed for a single environment.