chore(examples): update app title from Sample App to Flutter

Author: fadi-george

examples/build.md

@@ -99,7 +99,7 @@ Build the app with:
 - Dart 3+ with null safety
 - Material 3 theming with OneSignal brand colors
 - App name: "OneSignal Demo"
-- Top app bar: centered title with OneSignal logo SVG + "Sample App" text (use centerTitle: true on AppBar)
+- Top app bar: centered title with OneSignal logo SVG + "Flutter" text (use centerTitle: true on AppBar)
 - Support for both Android and iOS
 - Android package name: com.onesignal.example
 - iOS bundle identifier: com.onesignal.example

examples/demo/lib/screens/home_screen.dart

@@ -82,7 +82,7 @@ class _HomeScreenState extends State<HomeScreen> {
             ),
             const SizedBox(width: 8),
             Text(
-              'Sample App',
+              'Flutter',
               style: Theme.of(context).textTheme.bodyMedium?.copyWith(
                     color: Colors.white,
                   ),