v1.6.0

v1.6.0 🎬 Downloadable Windows/macOS app + release CI

- Frozen-aware path layer: a packaged build splits read-only bundled resources
  from a writable user-data dir (%LOCALAPPDATA%\Understudy /
  ~/Library/Application Support/Understudy). Source/dev behaviour is unchanged.
- Bundles the frontend, assets and the three default models (depth-int8,
  TopFormer, YOLOX-tiny) and seeds them into the writable dir on first run:
  zero-download first launch for depth + layout (pose still fetches once).
- PyInstaller onedir spec + model-fetch helper; freeze-safe desktop entry
  (freeze_support, run the app object under uvicorn). Built + launched on Windows.
- GitHub Actions builds and attaches the Windows + macOS app to the Release on a
  v* tag.
- README: "Download & run" section, refreshed feature screenshots, and a new
  shot-management figure (storyboard + whiteboard, diagonal merge).