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

api/node_ops.py

@@ -135,7 +135,6 @@ async def probe_node(node_id: str) -> dict[str, Any]:
             await _log(node_id, "info", "shell", "  WebSocket: /api/ws/ops")
 
         elif node_id in ("mo-commander", "bart-commander", "mcp-coordinator", "network-watcher"):
-            from node_registry import NODE_REGISTRY
             meta = NODE_REGISTRY[node_id]
             await _log(node_id, "ok", "shell", f"{meta['label']} online — monitoring all agent comms")
             await _log(node_id, "info", "shell", meta.get("description", ""))