Announcement banner

Site-wide notice bar — the first thing on the page, above the header or hero and in normal flow, never over the content. Full-bleed and square so it meets the viewport edges, rendered as a named aside. Text and one action is the whole shape: `href` and `action` are mutually exclusive at the type level, since a button inside an anchor is invalid HTML. `backgroundImage` adds a photo with a scrim and light text. Dismissal is a callback and `open` is the whole state — decide it server-side from a cookie so a dismissed bar never flashes.