feat(ads): add placeholder ad widget for unsupported platforms

- Create a new widget that displays a generic placeholder for advertisements
- Useful for maintaining UI consistency on platforms without native ad SDK support
- Features an icon, title, and subtitle to indicate it's an ad placeholder

Author: fulleni

lib/ads/widgets/placeholder_ad_widget.dart

@@ -0,0 +1,53 @@
+import 'package:flutter/material.dart';
+import 'package:ui_kit/ui_kit.dart';
+
+/// {@template placeholder_ad_widget}
+/// A widget that displays a generic placeholder for an advertisement.
+///
+/// This is used on platforms (like web) where actual native ad SDKs are not
+/// supported, but a visual representation of an ad slot is desired for UI
+/// consistency or demo purposes.
+/// {@endtemplate}
+class PlaceholderAdWidget extends StatelessWidget {
+  /// {@macro placeholder_ad_widget}
+  const PlaceholderAdWidget({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Container(
+      padding: const EdgeInsets.all(AppSpacing.lg),
+      decoration: BoxDecoration(
+        color: theme.colorScheme.surfaceVariant,
+        borderRadius: BorderRadius.circular(AppSpacing.md),
+        border: Border.all(color: theme.colorScheme.outlineVariant),
+      ),
+      child: Column(
+        mainAxisAlignment: MainAxisAlignment.center,
+        children: [
+          Icon(
+            Icons.ad_units,
+            size: 48,
+            color: theme.colorScheme.onSurfaceVariant,
+          ),
+          const SizedBox(height: AppSpacing.md),
+          Text(
+            'Ad Placeholder',
+            style: theme.textTheme.titleMedium?.copyWith(
+              color: theme.colorScheme.onSurfaceVariant,
+            ),
+            textAlign: TextAlign.center,
+          ),
+          const SizedBox(height: AppSpacing.xs),
+          Text(
+            'Native ads are not supported on this platform.',
+            style: theme.textTheme.bodySmall?.copyWith(
+              color: theme.colorScheme.onSurfaceVariant.withOpacity(0.7),
+            ),
+            textAlign: TextAlign.center,
+          ),
+        ],
+      ),
+    );
+  }
+}