Add simple desktop app

desktop/lib/main.dart

@@ -0,0 +1,22 @@
+import 'package:flutter/widgets.dart';
+import 'package:musicus_common/musicus_common.dart';
+import 'package:path/path.dart' as p;
+import 'package:path_provider/path_provider.dart' as pp;
+
+import 'settings.dart';
+import 'platform.dart';
+import 'playback.dart';
+
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+
+  final dir = await pp.getApplicationSupportDirectory();
+  final dbPath = p.join(dir.path, 'db.sqlite');
+
+  runApp(MusicusApp(
+    dbPath: dbPath,
+    settingsStorage: SettingsStorage(),
+    platform: MusicusDesktopPlatform(),
+    playback: MusicusDesktopPlayback(),
+  ));
+}