Add swingmusic-mobile submodule to replace Android app

- Updated .gitmodules to include mobile app submodule
- Added swingmusic_mobile directory with Flutter app
- Mobile app will now be built in unified release workflow

swingmusic_mobile/test/widget_test.dart

@@ -0,0 +1,21 @@
+// This is a basic Flutter widget test.
+//
+// To perform an interaction with a widget in your test, use the WidgetTester
+// utility in the flutter_test package. For example, you can send tap and scroll
+// gestures. You can also use WidgetTester to find child widgets in the widget
+// tree, read text, and verify that the values of widget properties are correct.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:swingmusic_mobile/main.dart';
+
+void main() {
+  testWidgets('SwingMusic App smoke test', (WidgetTester tester) async {
+    // Build our app and trigger a frame.
+    await tester.pumpWidget(const SwingMusicApp());
+
+    // Verify that the app loads
+    expect(find.byType(MaterialApp), findsOneWidget);
+  });
+}