feat: add React/Vite PWA web UI with FastAPI backend

- FastAPI + uvicorn server (turnup-ui) serving config API and static files
- React + TypeScript frontend: connection, global LEDs, per-knob/button
  cards, preset save/load/apply, toast notifications, dark theme
- Vite builds into src/turnup/ui/static/ so no Node.js needed at install time
- PWA assets (manifest, service worker, icon) in ui/public/
- turnup-ui.service systemd user unit; enabled globally via turnupd.install
- PKGBUILD: add python-fastapi + python-uvicorn deps, install UI service file
- pyproject.toml: ui extras group, turnup-ui entry point

.gitignore

@@ -37,7 +37,8 @@ pkg/
 # User config (not tracked — see contrib/config.example.json)
 config.json
 
-# IDE / editor
+# Node / Vite
+ui/node_modules/
 .vscode/
 .idea/
 *.swp