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/index.html

@@ -0,0 +1,15 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <link rel="icon" type="image/svg+xml" href="/icon.svg" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta name="theme-color" content="#0d1117" />
+    <link rel="manifest" href="/manifest.json" />
+    <title>TurnUp</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/main.tsx"></script>
+  </body>
+</html>