feat(announcements): move the banner to the chat composer, on whichever side it leaves free

[philmerrell]

Follow-up to #979. The banner sat at the top of the shell. What it announces — a new model, a new capability — is acted on in the composer, so the notice now lives where the decision is made rather than in a corner the eye has already left. It mounts from chat-input beside quota-warning-banner, which is where users already look for ambient notices.

It follows the composer, and the composer moves

A conversation pins the composer to the bottom of the viewport, where a pill below it would be off the edge. The empty state centres it with the greeting immediately above, where a pill above it floats over that greeting — visibly so at 375px, where the greeting wraps and the pill covered its second line.

So placement switches: below on the empty state, above otherwise.

Derived, not measured. isEmptyState() is the same computed that already picks which layout branch renders — the centred composer or the bottom-pinned one — so reading it makes the two impossible to drift apart. Measuring the composer's viewport position would re-derive that same fact less reliably, and would have to be recomputed on resize, on scroll, and when the artifact pane opens.

It still floats

bottom-full / top-full against a relative chat-input host keeps it clear of the quota tabs, which stay attached to the input, and preserves the property from #979 — measured before and after a dismissal:

	Δtop	Δleft	Δheight
app-chat-input	0	0	0
greeting	0	0	0
#app-scroll-container	0	0	0

Restyled to match the sibling it now sits beside — a compact shrink-to-fit pill (inline-flex, text-xs) rather than a bar spanning the composer.

The consequence worth agreeing on

The banner is now a chat-view surface only. It no longer appears on admin pages, settings, agents, or anywhere without a composer. That is deliberate — What's New remains the everywhere-record, which is why panel is forced onto every announcement server-side — but it is a real narrowing and worth a second opinion.

Rather than leave the design doc describing a placement that no longer exists, §D1 now says what shipped and why, and the two admin help strings that told authors "a strip below the top nav" now describe where a banner actually appears. An author choosing the banner surface should not be told the wrong thing.

Embedded panes are gated off

chat-input is reused by the agent-preview and marketplace test-drive panes, where a platform-wide "new models are available" pill would read as a bug rather than an announcement. A showAnnouncements input gates it, following the same opt-out shape as the show* controls beside it: default true, explicitly false at those two call sites, and threaded through chat-container so its embedded mode is off too.

The shell mount and its isAuthenticated() gate are gone with it — the composer only exists inside an authenticated chat route, so the 401-on-login hazard that gate existed for is now structural rather than guarded.

Verification

Browser-verified against dev data with a local app-api: light, dark, desktop and 375px, no horizontal overflow, dismiss still writing its durable server-side ack. On the empty state the pill sits below the composer in clear space and coversGreeting is false at both widths, where it was true before this change.

Three real test failures surfaced and were fixed, none of which a typecheck would have caught: two chat-container specs stub app-chat-input and needed the new inputs added to the stub (NG0303: Can't bind to 'showAnnouncements'), and one of the banner's own assertions was stale after the restyle. Full frontend suite 2490 passed on a clean run — including the submission-review.page.spec.ts spec that has been flaking.

One case not verified in a browser: the above placement inside a real conversation. The local stack has no inference-api, so I could not produce a loaded session; both host-class branches are covered by unit tests and the above styling was verified live on the empty state before this commit flipped it. Worth a glance on dev.

Independent of any other open PR — no shared files.

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