feat: read-only live view at /view?key=<LIVEVIEW_KEY>

Implements issue #5. Off by default; set LIVEVIEW_KEY in .env to enable.
No JWT required — key-based auth via ?key= query param.
Returns 403 when disabled or key is wrong.
Read-only ReactFlow canvas (pan/zoom, no editing).
Standalone mode loads from localStorage without a key.
Includes 8 backend tests and 9 frontend tests.

frontend/src/main.tsx

@@ -2,9 +2,12 @@ import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
 import './index.css'
 import App from './App.tsx'
+import LiveView from './components/LiveView.tsx'
+
+const isLiveView = window.location.pathname === '/view'
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
-    <App />
+    {isLiveView ? <LiveView /> : <App />}
   </StrictMode>,
 )