macOS native app v0.4.0-alpha.1: Tauri shell, runtime pack, MPS support, CI #60

Introduce a native macOS app built with Tauri v2:

• Tauri shell wrapping the Python/FastAPI backend
• Bundled portable Python runtime pack (tar.zst)
• Apple Silicon MPS acceleration for Demucs inference
• CI pipeline for macOS builds

Resolved by PR #27.