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/src/types.ts

@@ -223,6 +223,7 @@ export type PresentationSlide = {
   subtitle?: string
   bullets: string[]
   kind?: string
+  image?: string
   animation?: string
   topology?: TopologyViewData
   zone?: WorkloadZone