0.37.0: download whole folders (recursive) as a .zip from the file browser

The file browser/editor could only download individual files. Add a
recursive directory download that streams the folder as a zip archive,
on the local host and on every remote agent.

- file_service.archive_dir(): zip a directory recursively into a temp
  file, preserving the folder name as the archive root and empty
  subdirectories; symlinks are skipped (no sandbox escape / loops).
- /api/files/download and /agent/files/download branch on directories
  and return application/zip, cleaning up the temp file afterwards.
- Files page: show the download button for folders too (as <name>.zip).

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

backend/agent_app.py

@@ -33,6 +33,7 @@ from fastapi import (
     WebSocketDisconnect,
 )
 from fastapi.responses import FileResponse, JSONResponse
+from starlette.background import BackgroundTask
 from pydantic import BaseModel
 
 from config import settings
@@ -666,6 +667,12 @@ def files_read(path: str = Query(...)) -> dict:
 
 @app.get("/agent/files/download", dependencies=[Depends(verify_token)])
 def files_download(path: str = Query(...)):
+    if _file_guard(file_service.is_dir, path):
+        tmp, filename = _file_guard(file_service.archive_dir, path)
+        return FileResponse(
+            tmp, filename=filename, media_type="application/zip",
+            background=BackgroundTask(os.unlink, tmp),
+        )
     real, filename = _file_guard(file_service.resolve_download, path)
     return FileResponse(real, filename=filename, media_type="application/octet-stream")