feat(hub): cap a directory zip at 512 MB

An arbitrary ceiling, not a technical one: the zip writer streams and holds
one chunk plus a record per file, so it would happily produce a hundred
gigabytes. Past half a gigabyte the honest answer is a subfolder at a time,
or the files individually.

Enforced in file-utils.js's downloadDirectory, which is the one
implementation behind every zip button — Files' single folder, Files'
multi-folder selection, and the Photos album button (docs/photos.md §3).

- Per directory, not per selection: Files zips a whole multi-directory
  selection in one click, so an oversized folder is refused and its siblings
  still download.
- Before _openDownloadTarget, so no save dialog opens for an archive that is
  never going to be written.
- The bound is strict, so a folder of exactly 512 MB still goes through.
- Counted in the 1024-based units formatSize already prints, so the number in
  the refusal is the number in the constant.

group.zip_too_large in all ten catalogues. test_zip_size_limit.py runs the
module under Node and pins the refusal, the inclusive bound, and that nothing
is asked or started when a folder is over.

The user guide's "a 40 GB folder costs 40 GB of disk" is no longer true and
now documents the cap instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V8EDjk6pkYZrCbo63m2x87

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