Add HDFS browser, SSH terminal, presentation editor, lab health panel

- HDFS WebHDFS file browser (api/hdfs_api.py + HdfsView)
- In-browser SSH terminal via paramiko WebSocket bridge (api/ssh_terminal.py + SshTerminal, xterm.js)
- Presentation deck editor (text + image upload) and CRUD endpoints
- Collapsible GPU matrix + new LabHealthPanel in SideNav
- Topology fixes (edge alignment, Hadoop node, compact nodes)
- nginx ws timeout bump for long-lived SSH sessions

ui/nginx.conf

@@ -9,6 +9,9 @@ server {
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection "upgrade";
         proxy_set_header Host $host;
+        proxy_read_timeout 86400s;
+        proxy_send_timeout 86400s;
+        proxy_buffering off;
     }
 
     location /assets/ {