fix(ads): banner retries load when SDK becomes ready

On a cold start the consent flow is still running when home first builds, so
createBanner returns null and the slot stayed empty until a rebuild. AdService
now exposes an isReady ValueNotifier; BannerAdSlot listens and retries its load
once MobileAds finishes initializing. Verified: analyze clean, 169 tests green.

lib/services/ad_service.dart

@@ -1,6 +1,7 @@
 // lib/services/ad_service.dart
 import 'dart:async';
 
+import 'package:flutter/foundation.dart';
 import 'package:google_mobile_ads/google_mobile_ads.dart';
 
 import 'ad_config.dart';
@@ -29,11 +30,18 @@ class AdService {
   RewardedAd? _rewarded;
   bool _initialized = false;
 
-  /// Called by ConsentService once the SDK is initialized. Preloads ads.
+  /// Flips true once the SDK is initialized. Banner slots that were built
+  /// before consent finished listen to this and retry their load — otherwise
+  /// the banner would stay empty until the screen is rebuilt.
+  final ValueNotifier<bool> isReady = ValueNotifier<bool>(false);
+
+  /// Called by ConsentService once the SDK is initialized. Preloads ads and
+  /// notifies any waiting banner slots.
   void onSdkReady() {
     _initialized = true;
     _loadInterstitial();
     _loadRewarded();
+    isReady.value = true;
   }
 
   // ---- lifecycle hooks the game calls ----
@@ -137,5 +145,6 @@ class AdService {
   void dispose() {
     _interstitial?.dispose();
     _rewarded?.dispose();
+    isReady.dispose();
   }
 }