Phase 14: multi-host file browser (0.15.0)

The Files page gained a host switcher: when agents are registered, a Host
dropdown switches the whole browser between the local host and any online agent
(switching resets path + clipboard). Every file operation is sandboxed by the
selected agent's own ALLOWED_BROWSE_ROOTS/HOST_ROOT_PREFIX.

- agent_app.py: /agent/files/* (list/read/download/write/mkdir/touch/rename/
  copy/move/delete/upload) reusing file_service + device_service; BrowseError
  -> HTTP 400.
- routers/agents.py: proxy routes at /api/agents/{id}/files/* (audit-logged
  mutations); download streams via download_to_file, upload via upload_file.
  Reuses the WriteBody/NameBody/RenameBody/TransferBody models from routers.files.
- Frontend: filesApi methods take an optional trailing agentId; Files.tsx tracks
  a host and threads it through every call, query key, and the editor/dialogs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -8,7 +8,7 @@ as intuitive as Dockge, as capable as Portainer for Compose workflows.
 > destinations) + Phase 7 (Scheduled backups) + Phase 8 (Remote-stack backups)
 > + Phase 9 (Networks) + Phase 10 (iGPU passthrough) + Phase 11 (Remote UX &
 > network attach) + Phase 12 (File browser) + Phase 13 (Multi-host networks &
-> images) complete.
+> images) + Phase 14 (Multi-host file browser) complete.
 
 ## What works today (Phase 1)
 
@@ -146,6 +146,16 @@ as intuitive as Dockge, as capable as Portainer for Compose workflows.
   container or connect any container on the host (`POST /api/networks/{id}/connect`
   / `/disconnect`).
 
+### Phase 14 — Multi-host file browser
+
+- **The Files page now has a host switcher.** When agents are registered, a
+  *Host* dropdown at the top switches the whole browser between the local host
+  and any online agent; switching resets the path and clipboard.
+- All file operations (browse, view/edit, create, rename, copy/move, delete,
+  upload files & folders, download) work against the selected agent, sandboxed
+  by *that agent's* `ALLOWED_BROWSE_ROOTS`/`HOST_ROOT_PREFIX`.
+- New agent endpoints `/agent/files/*`, proxied at `/api/agents/{id}/files/*`.
+
 ### Phase 13 — Multi-host networks & images
 
 - **Networks and Images are now per-host.** Both pages render a section for the