Add client billing feature module

Introduces the client billing feature with domain, data, and presentation layers. Adds billing BLoC, navigation, UI widgets, and integrates with mock financial repository. Updates localization files for billing-related strings, adds new icons and typography, and registers url_launcher plugins for Linux, macOS, and Windows platforms.

apps/mobile/packages/features/client/client_main/pubspec.yaml

@@ -25,6 +25,9 @@ dependencies:
     path: ../home
   view_orders:
     path: ../view_orders
+  billing:
+    path: ../billing
+
   # Intentionally commenting these out as they might not exist yet
   # client_settings:
   #   path: ../settings