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/analysis_options.yaml

@@ -0,0 +1,12 @@
+include: package:flutter_lints/flutter.yaml
+
+analyzer:
+  exclude:
+    - build/**
+    - lib/theme/tokens.dart  # generated; allow magic numbers
+
+linter:
+  rules:
+    avoid_print: true
+    prefer_const_constructors: true
+    sort_pub_dependencies: false