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/lib/app.dart

@@ -0,0 +1,15 @@
+import 'package:flutter/material.dart';
+
+class MinstrelApp extends StatelessWidget {
+  const MinstrelApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      title: 'Minstrel',
+      home: Scaffold(
+        body: Center(child: Text('Minstrel — scaffold')),
+      ),
+    );
+  }
+}

flutter_client/lib/main.dart

@@ -0,0 +1,8 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'app.dart';
+
+void main() {
+  runApp(const ProviderScope(child: MinstrelApp()));
+}