docs(spa): say that a download with no folder cannot be paused

Reported from testing 7a: pause worked in the desktop app and no button
appeared in Chrome. That is the design working — without a granted download
folder, "save automatically" means the service worker, and that target is a
download the browser already owns — but nothing anywhere said so, and choosing
a folder looked like a question of where files land.

So the Settings line now says what it costs not to choose one, in all ten
catalogues. It renders only where a folder can be chosen at all, which is
exactly the browsers the advice applies to.

Also pins the tier table the pause button is drawn from: a granted folder, a
save dialog and the desktop sink can be paused, a service-worker stream cannot.
Four cases through the real `_openDownloadTarget`, and one more that reads the
value off the real `downloads.js` rather than a stub of it -- the first version
of these stubs did not carry the field at all, so the cases would have passed
while checking nothing.

Hub suite 847 passed.

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

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