fix(mobile): hide the Flutter debug banner (#8915)

Debug and simulator builds of the mobile app draw Flutter's red DEBUG
ribbon over the top-right corner of every screen. It covers app UI in
demo videos and screenshots, and nobody relies on it. This turns it off
on the app's `MaterialApp`, matching the profile avatar sheet app, which
already does. Release builds never showed it, so they're unchanged.

### Validation

- `flutter analyze` and `dart format` pass.
- Recorded the agent card flow demo on an iPhone 17 Pro simulator (iOS
27) from a debug build with this flag set: no DEBUG ribbon on any
screen.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Tom Brow <tomb@squareup.com>
Co-authored-by: Tom Brow <tomb@squareup.com>

mobile/lib/app.dart

@@ -348,6 +348,7 @@ class App extends HookConsumerWidget {
       navigatorKey: _mobileRootNavigatorKey,
       navigatorObservers: [appRouteObserver],
       title: 'Buzz',
+      debugShowCheckedModeBanner: false,
       theme: mobileAppearanceTheme(Brightness.light, communityTheme.accent),
       darkTheme: mobileAppearanceTheme(Brightness.dark, communityTheme.accent),
       themeMode: themeMode,