fix(iap): eagerly start IAP service at launch (final review I-1)

iapServiceProvider was lazy and only read by Settings, so the purchase stream
attached only after opening Settings — deferred/interrupted/restored
transactions wouldn't be delivered or completed until then. Read it in the
post-frame callback so the stream is live for the whole session.

lib/app.dart

@@ -19,6 +19,10 @@ class _BlockSeasonsAppState extends ConsumerState<BlockSeasonsApp> {
     super.initState();
     WidgetsBinding.instance.addPostFrameCallback((_) {
       ref.read(consentServiceProvider).ensureConsentAndInitialize();
+      // Eagerly start the IAP service so its purchase stream is live for the
+      // whole session — restores and interrupted/deferred transactions are
+      // delivered (and completed) even if the player never opens Settings.
+      ref.read(iapServiceProvider);
     });
   }