feat: add admin panel at /admin for full dashboard configuration

- Backend: ConfigManager write methods (atomic YAML save), admin CRUD
  router for settings (display/odoo/refresh) and nodes, WS broadcast
  on config changes, fix nmap scan blocking event loop with to_thread
- Frontend: admin UI with tab navigation, overview dashboard, node
  CRUD table with modal form, Odoo/display/refresh settings pages,
  typed API wrappers, active views filtering, config_changed WS handler
- Infra: nginx no-cache headers for HTML, cache-forever for hashed assets
- Fixes: WebSocket reconnect loop (ref pattern), rotation index OOB
  when views shrink, mutable node list cache

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -20,6 +20,7 @@
   "dependencies": {
     "framer-motion": "^12.34.0",
     "react": "^19.2.4",
-    "react-dom": "^19.2.4"
+    "react-dom": "^19.2.4",
+    "react-router-dom": "^7.13.0"
   }
 }