fix(spa): repair a page the download worker cannot serve

Downloads on Firefox failed with "the worker did not answer the download within
15s", every time, for one operator, while the same profile driven from here
succeeded every time. Their own test sequence found it: a freshly started
browser downloaded four files out of four, twice; one Ctrl+F5 and every
attempt afterwards failed; restart, fine again; Ctrl+F5 before any attempt and
the very first one failed.

A document fetched by a hard reload is loaded with the service worker bypassed.
It can still be claimed afterwards, so `navigator.serviceWorker.controller`
comes back and every check in `_claimController` passes — but the navigations
that document starts keep missing the worker, and the hidden iframe a streamed
download needs is a navigation. On Firefox and Safari that is the only way to
write a file too large to hold in memory, so the download cannot happen at all,
for the life of the page.

Being controlled is not being servable, so priming now asks the question
directly instead of inferring it: a four-byte stream and a hidden iframe,
exactly as a real download would, torn down completely so nothing lands in the
download folder. When it goes unanswered the page reloads once, ordinarily,
which puts it back under the worker. The flag lives in sessionStorage rather
than a variable because it has to survive the reload it triggers, and because a
page that is still unservable afterwards must stop rather than loop.

Also stops telling people to change browser. The message said "use the desktop
app, or Chrome or Edge" for a state an ordinary reload undoes, on the one path
Firefox has no alternative to; all ten catalogues now say to reload first.

The hard reloads were on my instruction: the SPA's HTML is served `no-store`,
so a plain reload has always picked up a new build and Ctrl+F5 was never needed.

Hub suite 834 passed.

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

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