feat(files): download a folder as a zip, and remove an empty one

Two things a Files panel needs and did not have.

**Removing a directory** is privileged, where creating one is not: it
acts on a name other members are using, on the operator's disk. It is
refused unless the directory is empty, and that rule is the safety
property — whatever the browser sends, this cannot destroy content. The
check runs twice, once before the challenge and once after the signature
comes back, because a file can land during the round trip. A file also
accepts its uploader's key; a directory has no uploader, so only the
operator's key will do.

**Downloading a folder** produces a zip built in the browser, written
straight to disk as the chunks arrive. An archive of a group folder is
routinely tens of gigabytes, so nothing is held: peak memory is one chunk
plus a small record per file. The node is not involved at all — it serves
the same encrypted chunks as any other download, holds no temporary
files, and cannot be asked to compress anything.

zipstream.js is store-only. Group content is video and images, already
compressed, so deflate would spend CPU on every byte to save nothing, in
the thread that is also decrypting. Sizes and CRCs go in a data
descriptor after each file because a stream cannot seek back to patch a
header, and zip64 kicks in per entry past 4 GiB and for the archive
itself. Because none of that can be checked from the Python side of the
house, test_zipstream.py runs the real module under Node and reads what
it produces with zipfile — CRCs, UTF-8 names, zip64 records and all. The
archives also pass `unzip -t`.

Firefox and Safari have no File System Access API, so there is nowhere to
stream to: the fallback builds the archive in memory and says so, with
the size, before starting rather than after failing.

One mistake worth recording: the first version of deleteDirectory passed
the node's own answer as the value to check the challenge against, which
turns the comparison into a tautology. It checks the path we asked for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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