feat(ads): add demo interstitial ad dialog widget

- Create a new dialog widget for displaying a placeholder interstitial ad in demo mode
- Mimics a full-screen ad with static text indicating it's a demo
- Includes a close button to dismiss the dialog

Author: fulleni

lib/ads/widgets/demo_interstitial_ad_dialog.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+import 'package:ui_kit/ui_kit.dart';
+
+/// {@template demo_interstitial_ad_dialog}
+/// A dialog widget that displays a placeholder for an interstitial ad in demo mode.
+///
+/// This dialog mimics a full-screen interstitial ad but contains only static
+/// text to indicate it's a demo.
+/// {@endtemplate}
+class DemoInterstitialAdDialog extends StatelessWidget {
+  /// {@macro demo_interstitial_ad_dialog}
+  const DemoInterstitialAdDialog({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Dialog.fullscreen(
+      backgroundColor: theme.colorScheme.surface,
+      child: Stack(
+        children: [
+          Center(
+            child: SingleChildScrollView(
+              child: Column(
+                mainAxisAlignment: MainAxisAlignment.center,
+                children: [
+                  Text(
+                    'INTERSTITIAL AD (DEMO)',
+                    style: theme.textTheme.titleLarge?.copyWith(
+                      color: theme.colorScheme.onSurface,
+                    ),
+                    textAlign: TextAlign.center,
+                  ),
+                  const SizedBox(height: AppSpacing.md),
+                  Text(
+                    'This is a full-screen advertisement placeholder.',
+                    style: theme.textTheme.bodyMedium?.copyWith(
+                      color: theme.colorScheme.onSurfaceVariant,
+                    ),
+                    textAlign: TextAlign.center,
+                  ),
+                ],
+              ),
+            ),
+          ),
+          Positioned(
+            top: AppSpacing.lg,
+            right: AppSpacing.lg,
+            child: IconButton(
+              icon: Icon(Icons.close, color: theme.colorScheme.onSurface),
+              onPressed: () {
+                // Dismiss the dialog.
+                Navigator.of(context).pop();
+              },
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}