fix: demo filebrowser uploads — increase nginx body size limit and add mock handlers

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

neode-ui/docker/nginx-demo.conf

@@ -61,10 +61,12 @@ http {
 
         # Proxy FileBrowser API to mock backend (demo mode)
         location /app/filebrowser/ {
+            client_max_body_size 10G;
             proxy_pass http://neode-backend:5959;
             proxy_http_version 1.1;
             proxy_set_header Host $host;
             proxy_set_header X-Real-IP $remote_addr;
+            proxy_request_buffering off;
         }
 
         # Serve AIUI SPA