web: replace Dialog, useConfirm and the useDialog* hooks with MUI Dialog #827

Part of #674. ~640 lines: Dialog.tsx, useConfirm, and the four useDialog* hooks.

Carry the evidence from the spike

Branch spike/674-base-ui ported Dialog onto a headless primitive on 2026-09-13: 338 -> 240
lines, typecheck clean, all style guards green — and 82 tests red across 16 files.

Nearly all of those failures asserted the old mechanism rather than a broken guarantee:
body.style.overflow === "hidden", inert on the container, aria-modal present. A probe
confirmed the behaviour itself was intact — focus landed on the first field, the background left the
accessibility tree — the library simply reaches the same end differently.

So the risk here is not the port. It is the test rewrite. Rewriting a guard to match a new
mechanism is exactly where an earned guarantee gets silently lost.

Required of this slice

Every rewritten test names the issue whose guarantee it protects:

• Sales: both dialogs can be open at once, so the untagged dialog-error slot misattributes #480 — everything outside the topmost dialog is out of the accessibility tree.
• Dialog: two open instances brick page scrolling, and Escape closes both — plus no background inertness #482 — page scroll lock survives stacked dialogs in any close order; one Escape closes one.
• fix(web): one page, one scroll lock and one live dialog (#482) #483 — focus restoration when a stacked dialog closes, without yanking a cursor already
  correctly placed in the dialog that stays open.
• PWA: the update banner's announcement is swallowed while a dialog is open #485 — anyDialogOpen/onModalStateChange, so a live region silenced under a dialog learns
  the page belongs to it again. Consumed by useMissedAnnouncement.
• fix: require step-up for flock scope changes #609 — closeDisabled suppresses every dismissal path mid-write.

inert and scroll lock cannot be settled in jsdom and need Playwright, per #501.