feat(spa): pause and resume a download, in session

Stage 7a of ~/next/improve-downloads.md: pausing within a session, on the
targets that can actually do it. Resuming across a reload is 7b.

A paused transfer holds **nothing**. Its slot goes back to the node the moment
it stops and resuming rejoins the queue at the tail, because anything else lets
one member close a node by pausing four downloads and going to lunch. So the
lease is taken inside the run loop rather than before it, and pause is refused
outright for a transfer that could not ask for another one.

Resuming is exact rather than approximate: the pipeline stops between two
chunks and never inside one, so what is on disk is always a whole number of
chunks and `fromChunk` is a verified position. The failure mode being avoided
is a file that looks complete and is quietly corrupt.

The target has to survive it, so a pause no longer reaches the `abort()` that a
failure does -- that would delete Electron's `.part` or the file just created in
the granted folder, leaving nothing to continue. And the in-memory fallback
keeps its accumulated chunks rather than starting a second array.

The button is drawn only where the target says it can. A service-worker stream
says no, in its own code and for its own reasons: the browser is already writing
an HTTP response into its own download folder, not feeding it stalls that
download where we cannot see or resume it, and an idle worker is terminated
within seconds. Firefox and Safari therefore keep cancel and get no pause, which
is the decision recorded in §6.5.

Cancelling a paused transfer ends it. A paused run is parked on a promise;
without waking it the row said "cancelled" over work that had not stopped and a
target that was still open.

Six cases, each checked against the unfixed source. Hub suite 842 passed.

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

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