feat(files): transfers that outlive the page, and selection instead of per-row menus

Downloads and uploads were state inside GroupPage. Leaving a group
unmounted the component, its cleanup closed the DataChannel, and a
half-written file was all you had — which is also why only one thing
could be in flight at a time.

They live in a module-level store now. A group page hands its transport
over on the way out rather than closing it, and the last transfer using
it closes it; signing out is the one thing that cancels everything,
because those transfers are moving data on a token about to stop being
ours. The store is plain JavaScript with no browser globals, so
test_transfers.py runs it under Node and pins the parts that are timing
and lifetime rather than markup: that a cancel stops the work instead of
greying out a row, that a stalled transfer reads as stalled rather than
reporting its own historical average, and that a released transport is
closed by the last transfer and not before.

The widget by the bell shows each transfer with its rate and a cancel
button, so the Files panel no longer carries progress bars — you can
watch a 40 GB archive from the chat, or from another group.

Selection replaces the per-row menu: a Select toggle puts checkboxes on
files and folders, and ⋮ Actions acts on what is ticked. Ticks survive
walking into another folder, so a selection can span directories.
Downloads start together and run together. Videos offer Play only — View
did the same thing, which is the sort of duplication that makes people
wonder what the difference is.

Uploads had to become parallel-safe for any of this to mean anything:
their acks were matched by arrival order, so two at once credited each
other's progress. The node names the file in every ack, so they are keyed
by name now — with the same file twice refused, since the node keys its
own upload state that way too.

Two mistakes worth recording. The selection column went into the body
rows and not the header, because that edit matched nothing and I had not
made it assert; the columns were misaligned until a screenshot showed it.
And the Actions menu opened leftwards from a button at the right edge of
the toolbar, half of it off-screen.

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

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