fix(spa): say why a download cannot be paused

Reported from Chrome, with a screenshot: four downloads with no pause button
and an upload beside them with one, and nothing anywhere saying why.

The reason is real. Without a granted download folder the browser writes
through the service worker — a download it already owns, which cannot be paused
without stalling it somewhere we can neither see nor resume. An upload writes to
the node, which keeps the position, so it is always pausable. But that was
stated only in a Settings line nobody reads on the way to a download, and a gap
where the row above has a button is not an explanation.

So a download that cannot be paused now shows a dimmed pause icon where the
button would be, carrying the reason and the remedy in its tooltip. Not a
button: there is nothing to click, and a disabled one invites the click anyway.

And only where the advice can be taken. Firefox and Safari have no folder to
choose — the streamed path is the only target they have, which is what §6.5 of
~/next/improve-downloads.md costs out — so telling someone there to choose one
would be advice they cannot follow. Nothing is drawn.

Hub suite 866 passed.

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

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