feat(notices): host-driven notice banners on the dashboard home

[camielvs]

Why

This is the PR that swaps the announcements system for the notices one. Everything below it was preparation; everything above it is optional.

What you get

AnnouncementBanners and src/config/announcements.ts are deleted. NoticeBanners takes the exact slot <AnnouncementBanners /> occupied — first child of DashboardHomeView's BlockStack, dashboard home only, nowhere else in the app.

Scope of this PR is announcement parity, deliberately. A reader can see banners and dismiss the ones the host marked dismissible. That's it. The notice inbox is #2668 and hiding is #2681, so each can be judged — or dropped — on its own.

What parity buys over the deleted version, all of it a consequence of #2666 rather than new surface here:

• an unrecognised variant no longer crashes the render
• a body renders as Markdown, so a notice can link and emphasise
• an optional action button
• notices appear when the host publishes them, without a reload, and a dismissal in one tab applies in the others
• NoticeBanners and useNotices are React Compiler-enabled; AnnouncementBanners could not be, because it read a mutable global and called new Date() in render

Two visible differences from the deleted banners, both intentional:

• Severity order. error → warning → success → info, rather than host array order, so an outage isn't below a nice-to-know.
• Three-column grid matching the dashboard's other rows, rather than full-width stacked. A long body scrolls inside its own card with the action pinned below it, so one verbose notice can't push the rest of the page down.

Reviewer notes

src/hooks/useNotices.ts is where the non-obvious code is:

Dismissal is two-tier, and this is the bit worth reading. dismissible: true from the host means "the reader may retire this permanently" → localStorage. A notice without dismissible is mandatory, so it can be cleared for the current session only and returns on the next load. In this PR the banners only ever offer the X on dismissible notices, so the session tier has no UI yet — it's what #2668's inbox and #2681's hide build on, and it is tested directly.

refresh is optional and this is the only caller. One visibilitychange listener, throttled to at most once per 30s, calling source.refresh?.(). Nothing in this feature polls on a timer — no setInterval is added anywhere. If the host doesn't implement refresh, this is a no-op and notices simply update whenever the host publishes.

Store rather than context. Module-level state + useSyncExternalStore, because two unrelated subtrees read it (the banners here, the header button in #2668) and neither owns the other. It also gives a stable snapshot to the React Compiler and picks up the cross-tab storage event for free.

Also here: notices.ts's TangleNoticeAction/TangleNoticeSource stop being exported now that nothing outside the module names them, and knip.json drops both src/config/announcements.ts and src/config/notices.ts from its ignore list — the first because the file is gone, the second because it now has real consumers.

How to test

Install a source in the console and dispatch the event (see src/config/NOTICES.md):

window.__TANGLE_NOTICE_SOURCE__ = {
  version: 1,
  getSnapshot: () => [
    { id: "1", title: "Scheduled maintenance", body: "Submissions paused **09:00–11:00 UTC**.", variant: "warning", dismissible: true },
    { id: "2", title: "Read-only mode", body: "", variant: "error" },
  ],
  subscribe: () => () => {},
};
window.dispatchEvent(new CustomEvent("tangle:notice-source"));

• Dashboard home shows both, error first, no reload needed.
• The warning has an X and stays gone after a reload; the error has none.
• No banners on any other route.
• With the global unset, the home page is byte-for-byte what it is on master with no announcements installed.

Where this sits

	PR
1	#2664	shared Markdown renderer
2	#2666	validated host contract
3	#2667	notice banners on the dashboard home — announcement parity (you are here)
4	#2668	notices button in the header
5	#2681	hide a banner without retiring it

This is the merge point. Stopping here is a complete, shippable replacement for the announcements system. #2668 and #2681 add surface on top and are independently droppable.

[camielvs]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(notices): let a reader hide a banner without retiring it #2681
• feat(notices): notices button in the header #2668
• feat(notices): host-driven notice banners on the dashboard home #2667 👈 (View in Graphite)
• feat(notices): validated contract for host-supplied notices #2666
• feat(markdown): shared Markdown renderer #2664 : 1 other dependent PR (#2665 )
• master

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