feat(hub): client-side transfer leases and the transfers panel

Steps 5 and 6 of ~/next/improve-downloads.md. The node has handed out slots
since step 2 and nothing asked for one; now the client does, and the panel shows
what is happening.

`transport.openTransfer()` returns a Lease: `acquire()` resolves when the node
grants, `release()` gives it back exactly once, and nothing else in the client
speaks to the node about slots. Whether a node hands out slots is read from the
handshake ack rather than guessed from a timeout — "no answer yet" and "this
node will never answer" are indistinguishable in time, and guessing wrong either
stalls every download or defeats the cap.

Two things exist only because a queue can lie: a watchdog re-asks when a pushed
grant does not arrive (the node is idempotent on `tr`, so asking again is free),
and a grant for a transfer the page has forgotten is handed straight back rather
than held until the node's deadline.

The slot is asked for **after** there is somewhere to write, and that ordering
is load-bearing: opening a target takes thirty seconds of streamed-download
timeouts, or as long as somebody leaves a Save As dialog open, and a grant not
taken up in time is revoked. Moving it earlier looked better and broke three
downloads into one. Pinned by a test.

The panel groups by state — running, waiting, finished — rather than re-sorting a
flat list, so a row moves only when its own state does. The ETA is withheld until
the speed window holds real measurement: a figure from the first two chunks
swings between four seconds and an hour, and people plan around the first number
they see. One live region announces state changes and not progress.

Three silent paths closed on the way: a download refused for want of a user
gesture (a browser grants one file picker per gesture, and downloading three
files is one gesture) now falls back to the streamed path, which needs none; a
click with no connection says so instead of doing nothing at all; and a queued
transfer counts as busy, so a transport is never closed under one that is
waiting for a grant that could then never arrive.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST

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