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/README.md

@@ -0,0 +1,17 @@
+# minstrel
+
+Minstrel mobile client
+
+## Getting Started
+
+This project is a starting point for a Flutter application.
+
+A few resources to get you started if this is your first Flutter project:
+
+- [Learn Flutter](https://docs.flutter.dev/get-started/learn-flutter)
+- [Write your first Flutter app](https://docs.flutter.dev/get-started/codelab)
+- [Flutter learning resources](https://docs.flutter.dev/reference/learning-resources)
+
+For help getting started with Flutter development, view the
+[online documentation](https://docs.flutter.dev/), which offers tutorials,
+samples, guidance on mobile development, and a full API reference.