feat(flutter): project scaffold with pubspec and smoke test

iOS + Android targets only; desktop disabled (per spec, Tauri-wrapped
SvelteKit handles desktop in v1.1). Riverpod + dio + just_audio +
audio_service + go_router pinned in pubspec. Smoke test confirms the
ProviderScope wiring boots a Material app.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/test/smoke_test.dart

@@ -0,0 +1,11 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/app.dart';
+
+void main() {
+  testWidgets('app boots and renders scaffold marker', (tester) async {
+    await tester.pumpWidget(const MaterialApp(home: MinstrelApp()));
+    expect(find.text('Minstrel — scaffold'), findsOneWidget);
+  });
+}