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/services/file_service.py

@@ -10,6 +10,8 @@ from __future__ import annotations
 
 import os
 import shutil
+import tempfile
+import zipfile
 
 from services.device_service import BrowseError, _is_allowed, _real_root
 
@@ -175,6 +177,47 @@ def resolve_download(path: str) -> tuple[str, str]:
     return real, os.path.basename(path)
 
 
+def is_dir(path: str) -> bool:
+    """Whether ``path`` points at a directory inside the sandbox."""
+    return os.path.isdir(_safe_real(path))
+
+
+def archive_dir(path: str) -> tuple[str, str]:
+    """Zip a directory (recursively) into a temp file.
+
+    Returns ``(tmp_zip_path, download_filename)``. The caller is responsible
+    for deleting the temp file once it has been streamed to the client.
+    Symlinks are skipped so the archive cannot escape the sandbox or loop.
+    """
+    real = _safe_real(path)
+    if not os.path.isdir(real):
+        raise BrowseError(f"Not a directory: {path}")
+    name = os.path.basename(path.rstrip("/")) or "root"
+
+    fd, tmp = tempfile.mkstemp(suffix=".zip")
+    os.close(fd)
+    try:
+        with zipfile.ZipFile(tmp, "w", zipfile.ZIP_DEFLATED) as zf:
+            for root, dirs, files in os.walk(real):
+                # Don't follow symlinked directories (avoids loops / escapes).
+                dirs[:] = [d for d in dirs if not os.path.islink(os.path.join(root, d))]
+                rel_root = os.path.relpath(root, real)
+                if not files and not dirs and rel_root != ".":
+                    # Preserve otherwise-empty directories.
+                    zf.writestr(os.path.join(name, rel_root) + "/", "")
+                for f in files:
+                    full = os.path.join(root, f)
+                    if os.path.islink(full):
+                        continue
+                    zf.write(full, os.path.join(name, rel_root, f) if rel_root != "."
+                             else os.path.join(name, f))
+    except OSError:
+        if os.path.exists(tmp):
+            os.unlink(tmp)
+        raise
+    return tmp, f"{name}.zip"
+
+
 def upload_target(
     dir_path: str,
     filename: str,