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

contrib/turnup-ui.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Turn Up web UI
+Documentation=https://github.com/sean351/turn-up-arch
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/turnup-ui
+Restart=on-failure
+RestartSec=5
+
+# Logging
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=turnup-ui
+
+[Install]
+WantedBy=default.target