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/public/icon.svg

@@ -0,0 +1,15 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <rect width="512" height="512" rx="80" fill="#0d1117"/>
+  <!-- Knob body -->
+  <circle cx="256" cy="210" r="110" fill="#161b22" stroke="#58a6ff" stroke-width="14"/>
+  <!-- Knob indicator line -->
+  <line x1="256" y1="210" x2="256" y2="122" stroke="#58a6ff" stroke-width="12" stroke-linecap="round"/>
+  <!-- Center dot -->
+  <circle cx="256" cy="210" r="18" fill="#58a6ff"/>
+  <!-- 5 dots = 5 knobs/buttons -->
+  <circle cx="116" cy="390" r="14" fill="#3fb950"/>
+  <circle cx="176" cy="390" r="14" fill="#3fb950"/>
+  <circle cx="256" cy="390" r="14" fill="#58a6ff"/>
+  <circle cx="336" cy="390" r="14" fill="#3fb950"/>
+  <circle cx="396" cy="390" r="14" fill="#3fb950"/>
+</svg>

ui/public/manifest.json

@@ -0,0 +1,17 @@
+{
+  "name": "TurnUp",
+  "short_name": "TurnUp",
+  "description": "Configure your TurnUp knob/button mixer",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0d1117",
+  "theme_color": "#0d1117",
+  "icons": [
+    {
+      "src": "/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}

ui/public/sw.js

@@ -0,0 +1,31 @@
+// Network-first SW: static assets are cached after first fetch;
+// API calls always go to the network.
+const CACHE = 'turnup-v1';
+
+self.addEventListener('install', () => {
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', (e) => {
+  e.waitUntil(
+    caches.keys().then((keys) =>
+      Promise.all(keys.filter((k) => k !== CACHE).map((k) => caches.delete(k)))
+    )
+  );
+  self.clients.claim();
+});
+
+self.addEventListener('fetch', (e) => {
+  // Always go network-first for API calls so the UI never serves stale data
+  if (e.request.url.includes('/api/')) return;
+
+  e.respondWith(
+    fetch(e.request)
+      .then((response) => {
+        const clone = response.clone();
+        caches.open(CACHE).then((c) => c.put(e.request, clone));
+        return response;
+      })
+      .catch(() => caches.match(e.request))
+  );
+});

ui/public/vite.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" aria-hidden="true" role="img" class="iconify iconify--logos" width="31.88" height="32" preserveAspectRatio="xMidYMid meet" viewBox="0 0 256 257"><defs><linearGradient id="IconifyId1813088fe1fbc01fb466" x1="-.828%" x2="57.636%" y1="7.652%" y2="78.411%"><stop offset="0%" stop-color="#41D1FF"></stop><stop offset="100%" stop-color="#BD34FE"></stop></linearGradient><linearGradient id="IconifyId1813088fe1fbc01fb467" x1="43.376%" x2="50.316%" y1="2.242%" y2="89.03%"><stop offset="0%" stop-color="#FFEA83"></stop><stop offset="8.333%" stop-color="#FFDD35"></stop><stop offset="100%" stop-color="#FFA800"></stop></linearGradient></defs><path fill="url(#IconifyId1813088fe1fbc01fb466)" d="M255.153 37.938L134.897 252.976c-2.483 4.44-8.862 4.466-11.382.048L.875 37.958c-2.746-4.814 1.371-10.646 6.827-9.67l120.385 21.517a6.537 6.537 0 0 0 2.322-.004l117.867-21.483c5.438-.991 9.574 4.796 6.877 9.62Z"></path><path fill="url(#IconifyId1813088fe1fbc01fb467)" d="M185.432.063L96.44 17.501a3.268 3.268 0 0 0-2.634 3.014l-5.474 92.456a3.268 3.268 0 0 0 3.997 3.378l24.777-5.718c2.318-.535 4.413 1.507 3.936 3.838l-7.361 36.047c-.495 2.426 1.782 4.5 4.151 3.78l15.304-4.649c2.372-.72 4.652 1.36 4.15 3.788l-11.698 56.621c-.732 3.542 3.979 5.473 5.943 2.437l1.313-2.028l72.516-144.72c1.215-2.423-.88-5.186-3.54-4.672l-25.505 4.922c-2.396.462-4.435-1.77-3.759-4.114l16.646-57.705c.677-2.35-1.37-4.583-3.769-4.113Z"></path></svg>