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/ui/widgets/banner_ad_slot.dart

@@ -3,11 +3,16 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:google_mobile_ads/google_mobile_ads.dart';
 
+import '../../services/ad_service.dart';
 import '../../state/providers.dart';
 
 /// A self-loading 320x50 banner for the home/map screens. Renders nothing
 /// when ads are removed or the banner has not loaded — never reserves blank
 /// space and never appears on the game screen.
+///
+/// If the slot is built before the AdMob SDK finished initializing (common on
+/// a cold start while the consent flow is still running), the first create
+/// returns null; it then retries once the [AdService.isReady] notifier flips.
 class BannerAdSlot extends ConsumerStatefulWidget {
   const BannerAdSlot({super.key});
 
@@ -18,13 +23,22 @@ class BannerAdSlot extends ConsumerStatefulWidget {
 class _BannerAdSlotState extends ConsumerState<BannerAdSlot> {
   BannerAd? _ad;
   bool _loaded = false;
+  AdService? _adService;
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    if (_ad != null) return;
+    if (_adService == null) {
+      _adService = ref.read(adServiceProvider);
+      _adService!.isReady.addListener(_tryCreate);
+    }
+    _tryCreate();
+  }
+
+  void _tryCreate() {
+    if (_ad != null || !mounted) return;
     if (ref.read(adsRemovedProvider)) return;
-    final ad = ref.read(adServiceProvider).createBanner(
+    final ad = _adService!.createBanner(
       listener: BannerAdListener(
         onAdLoaded: (_) {
           if (mounted) setState(() => _loaded = true);
@@ -32,12 +46,13 @@ class _BannerAdSlotState extends ConsumerState<BannerAdSlot> {
         onAdFailedToLoad: (ad, _) => ad.dispose(),
       ),
     );
-    if (ad == null) return;
+    if (ad == null) return; // SDK not ready yet; isReady will retry.
     _ad = ad;
   }
 
   @override
   void dispose() {
+    _adService?.isReady.removeListener(_tryCreate);
     _ad?.dispose();
     super.dispose();
   }