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>

.gitignore

@@ -28,3 +28,17 @@ go.work.sum
 
 # Superpowers brainstorming companion sessions
 .superpowers/
+
+# Flutter
+flutter_client/.dart_tool/
+flutter_client/.flutter-plugins
+flutter_client/.flutter-plugins-dependencies
+flutter_client/build/
+flutter_client/.idea/
+flutter_client/ios/Podfile.lock
+flutter_client/ios/Pods/
+flutter_client/android/.gradle/
+flutter_client/android/app/build/
+flutter_client/android/local.properties
+flutter_client/android/key.properties
+flutter_client/*.iml