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

ui/.gitignore

@@ -0,0 +1,24 @@
+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+lerna-debug.log*
+
+node_modules
+dist
+dist-ssr
+*.local
+
+# Editor directories and files
+.vscode/*
+!.vscode/extensions.json
+.idea
+.DS_Store
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?