feat(LINCHPIN-5639): Add a settings card, place notices and rename the stylesheet

[aaronware]

Everything here came out of adopting this package in psst, the first real consumer. Task: Standard admin component library: linchpin/ui.

The packaging fix, which every consumer needs

The compiled stylesheet moves to build-style/chrome.css. @wordpress/scripts routes any module whose file is named style.css into a separate style-<entry>.css — the mechanism that separates a block's front-end styles from its editor ones — so a library stylesheet with that name never reaches the bundle the plugin enqueues, and the screen loads with no chrome on it.

Because the rule matches the file rather than the specifier, import '@linchpinagency/ui/style.css' still works. No consumer changes an import or configures webpack. Verified end to end: packed this branch, installed the tarball into psst, deleted psst's workaround, built — one admin.css with the chrome in it.

New

• <SettingsCard> — the section shape every plugin was still building by hand out of a Card, a CardHeader holding a heading and a sentence, and a CardBody. The screen story in this repo was a third copy of it. Takes title, description, headingLevel, actions; renders no header without one, and no body without children.

Fixed

• lp-admin__notices is placed by the chrome rather than left to each consumer. Core's .components-snackbar-list is a full-width, click-through strip, so the strip is pinned and the snackbars centred inside it — centring the strip itself slides its whole width off to the left, which is where psst's snackbars ended up.
• The layout's middle column is a <div>, so it stops nesting a second main landmark inside the one wp-admin already opens as <div id="wpbody" role="main">.
• The danger zone takes the strong error stroke at 2px on all four sides. At 1px in the ordinary error colour it read as a card with a faintly pink header rather than the one section that can destroy something.
• The danger zone's standing warning is no longer announced on mount. Notice speaks its children, and an error status speaks them assertively, so it interrupted a screen reader on every page load to read a sentence already on screen.

Checks

lint:js, lint:css, build and verify:esm pass. 89 tests, 6 new — covering the card, the landmark and the silent warning. The announcement test fails without its fix; I checked.

Storybook has a new Chrome/Settings card story, and the screen story now uses the component instead of hand-rolling one. Worth a look there before merging: I verified the danger-zone border in psst rather than in Storybook.