[Feature]: Download large folders as a single archive (auto-zip above a configurable file count) #24319

Problem or use case

Download Folder in the File Explorer (#7780, #7793) works well for small folders but is impractical for folders with thousands of files (build output, datasets, logs, anything with node_modules or .venv inside):

• Files are copied one by one over SFTP, sequentially, so per-file round-trips dominate and a folder of many small files takes a very long time.
• One symlink or special file anywhere in the tree aborts the whole transfer (Cannot download symbolic link '…').
• Because the transfer lands in a temp folder that is discarded on failure, a download that fails late leaves nothing behind.

The workaround today is to leave the explorer: run tar czf ./folder.tar.gz folder in a terminal, download the archive from the explorer, then delete it on the remote.

Proposed solution

When a folder is downloaded and it contains more files than a configurable threshold, archive it on the remote host and transfer a single file instead of copying the tree.

1. Threshold check. Before transferring, count entries in the selected folder, stopping as soon as the count exceeds the threshold, so a huge tree costs only a few directory reads.
2. Setting. For example File Explorer › Archive folders before download when they contain more than N files, default 10. 0 = always archive; a toggle (or empty value) = never archive, keeping today's behavior.
3. Above the threshold: the save dialog suggests <folder>.zip, the remote side builds the archive, and the client downloads one file.
4. At or below the threshold: unchanged recursive copy.
5. Explicit action: a Download as .zip context-menu item on folder rows that always archives, regardless of the threshold.

Implementation notes / suggestions:

• No host dependency. Build the archive in the relay (Node) and stream it to the client, rather than shelling out to zip, which is often not installed on Linux servers. If a temp file on the host is needed, put it in the host's temp directory and remove it after success, failure or cancel.
• Symlinks are stored as links, not followed. They can no longer escape the selected tree or abort the download.
• Format: .zip by default, since it opens natively on macOS, Windows and Linux. .tar.gz could be an option for preserving permissions.
• Progress and cancel: reuse the transfer panel from feat(transfers): progress and cancel for SSH uploads and all remote downloads #23849. Cancel stops archiving on the host and removes the partial local file.
• Limits: keep the budgets from fix(ssh): bound remote directory listing and folder transfer #10878 (depth, per-file bytes, total bytes) so archiving cannot be used to bypass them.
• Runtime workspaces: because the result is a single file, this could also enable folder download for orca serve / runtime-hosted workspaces, which today support only single-file download.

Open question: should there be an optional "extract after download" so the user still ends up with a folder locally?

Alternatives or additional context

• Parallel SFTP transfers would speed up the current copy, but would not fix the symlink aborts or the all-or-nothing failure on huge trees.
• Manual workaround: tar/zip in the terminal, then download the archive. This works, but breaks the explorer flow and leaves cleanup to the user.

Related: #7780 (original request, which already mentioned "zipped automatically" as an option), #7793 (current recursive implementation), #10878 (transfer budgets), #23849 (transfer progress/cancel), #9031 (paired web Explorer transfers).