feat: add standalone (frontend-only) mode via VITE_STANDALONE build flag

- App.tsx: bypass auth gate, swap canvas save/load to localStorage
- Sidebar.tsx: hide scan views and Scan Network button
- useStatusPolling.ts: skip WebSocket connection
- Dockerfile.frontend: accept VITE_STANDALONE build arg
- docker-publish.yml: build ghcr.io/pouzor/homelable-frontend-standalone image
- docker-compose.standalone.yml: frontend-only compose file
- install.sh: add --standalone flag

Usage:
  curl -fsSL .../install.sh | bash -s -- --standalone
  cd homelable && docker compose up -d

frontend/src/hooks/useStatusPolling.ts

@@ -12,13 +12,15 @@ interface StatusMessage {
   devices_found?: number
 }
 
+const STANDALONE = import.meta.env.VITE_STANDALONE === 'true'
+
 export function useStatusPolling() {
   const wsRef = useRef<WebSocket | null>(null)
   const { updateNode, notifyScanDeviceFound } = useCanvasStore()
   const { isAuthenticated, token } = useAuthStore()
 
   useEffect(() => {
-    if (!isAuthenticated || !token) return
+    if (STANDALONE || !isAuthenticated || !token) return
 
     const protocol = window.location.protocol === 'https:' ? 'wss' : 'ws'
     const host = window.location.hostname