feat: complete centralized error handling system with documentation

apps/mobile/packages/features/client/billing/lib/src/presentation/pages/billing_page.dart

@@ -180,9 +180,16 @@ class _BillingViewState extends State<BillingView> {
     }
 
     if (state.status == BillingStatus.failure) {
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          SnackBar(content: Text(translateErrorKey(state.errorMessage!))),
+        );
+      });
       return Center(
         child: Text(
-          state.errorMessage ?? 'An error occurred',
+          state.errorMessage != null
+              ? translateErrorKey(state.errorMessage!)
+              : 'An error occurred',
           style: UiTypography.body1r.textError,
         ),
       );