feat: comprehensive React SPA replacing Jinja2 UI

- New dark UI with 7 tabs: Rig, FX Chain, Models, IRs, Presets, Settings, Record
- All screens wired to live API endpoints
- Real-time state via WebSocket + polling fallback
- Models screen: list/load/unload/upload NAM files + Tone3000 search/install
- IRs screen: list/load/unload/upload IR files + Tone3000 search/install
- Presets screen: browse banks, activate, save, delete
- Settings screen: WiFi scan/connect/hotspot, Bluetooth/MIDI, 4CM routing
- Rig screen: live state, volume, bypass, tuner, signal chain
- FX Chain screen: block params, per-block bypass
- Serves at / instead of /ui
- Built with Vite, ~71KB gzipped

frontend-react/src/index.css

@@ -0,0 +1,2 @@
+* { margin: 0; padding: 0; box-sizing: border-box; }
+body { background: #0A0A0C; color: #F0EDE6; }

frontend-react/src/main.jsx

@@ -0,0 +1,10 @@
+import { StrictMode } from 'react'
+import { createRoot } from 'react-dom/client'
+import './index.css'
+import App from './App.jsx'
+
+createRoot(document.getElementById('root')).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+)