feat: add mock FileBrowser API and WebSocket fixes for demo

- Mock FileBrowser endpoints: login, list directories, read text files
- Demo content: Music (17 tracks), Documents, Photos, Videos
- Proxy /app/filebrowser/ to backend in nginx-demo.conf
- Add node-messages-received RPC stub (stops console errors)
- WebSocket heartbeat every 45s (prevents 60s disconnect loop)

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

neode-ui/docker/nginx-demo.conf

@@ -59,6 +59,14 @@ http {
             proxy_set_header X-Real-IP $remote_addr;
         }
 
+        # Proxy FileBrowser API to mock backend (demo mode)
+        location /app/filebrowser/ {
+            proxy_pass http://neode-backend:5959;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+        }
+
         # Serve AIUI SPA
         location /aiui/ {
             alias /usr/share/nginx/html/aiui/;