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/ios/Runner.xcworkspace/contents.xcworkspacedata

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Workspace
+   version = "1.0">
+   <FileRef
+      location = "group:Runner.xcodeproj">
+   </FileRef>
+</Workspace>