add macOS support for ad loading and improve timestamp handling

lib/services/adsense_service_web.dart

@@ -13,13 +13,13 @@ class AdSenseService implements AdService {
   Future<void> initialize() async {
     // AdSense is loaded via script tag in index.html
     print("AdSense initialized (web)");
-    _isLoaded = true;
+    _isLoaded = false;
   }
 
   @override
   Future<void> loadBannerAd() async {
     // AdSense ads are loaded automatically when widget renders
-    _isLoaded = true;
+    _isLoaded = false;
   }
 
   @override