PWA: the update banner's announcement is swallowed while a dialog is open #485

#483 makes everything except the topmost dialog inert (that is what stops a second dialog being reachable, and what closes the a11y hole aria-modal only hints at). One consequence is worth deciding deliberately rather than inheriting.

UpdatePrompt (web/src/pwa/UpdatePrompt.tsx:61) is a persistent role="status" aria-live="polite" banner with Reload / Later buttons, mounted inline under #root (App.tsx:80) — a document.body child, so it goes inert whenever any dialog is open.

If a service-worker update lands while the user has a dialog open:

1. the banner is unclickable until the dialog closes, and
2. it is out of the accessibility tree, so its aria-live announcement is never made — and un-inerting later does not re-announce it.

(1) is arguably correct and worth keeping. Reloading mid-form would discard whatever the user is typing, so a Reload button that cannot be hit while a dialog is open is a feature. (2) is a real, if narrow, regression: a screen-reader user gets no notification at all, rather than a delayed one.

Found by an internal reviewer during #483. Not fixed there, because the options each have a cost worth weighing separately:

• Re-announce when the last dialog closes — smallest honest fix; needs UpdatePrompt to know the modal state, or a re-mount trigger.
• Exempt opted-in elements from inertness (data-modal-exempt) — general, but a reachable control outside the modal partly defeats the point, and the focus trap would fight it.
• Accept it and document — defensible; the banner is still there when the dialog closes, just never announced.

Also noted, lower risk: AppLayout.tsx:93-108's farm-load-failed role="alert" banner is inerted the same way, but it is already on screen before any dialog opens, so nothing is missed — only temporarily unclickable, which is the correct modal behaviour.

No test covers the combination today: UpdatePrompt.test.tsx renders it standalone, and tools/simulation/ui/specs/pwa.spec.ts never has a dialog open at the same time.