feat(webkit)!: reshape Message onto slot-first copy with label and size

[gabriel-lisboa-azion]

Summary

Message presented a title + description stack, so copy carrying an inline link had no home — the only escape was the default slot, which replaced the icon and the entire layout along with it. This reshapes the banner onto a single line of copy in one content region that keeps the leading icon and styles anchors with the .text-link token, and adds a size token driving the height floor, inline padding, copy scale, and the trailing control sizes.

Breaking: title (required) and description are gone. Pass the copy as label, or in the default slot when it needs inline markup.

<!-- before -->
<Message severity="warning" title="Once confirmed, this action can't be reversed." />

<!-- after -->
<Message severity="warning" label="Once confirmed, this action can't be reversed." />

<!-- copy with inline links -->
<Message severity="warning">
  Your plan is close to its request limit.
  <a href="https://github.com/billing">Upgrade the plan</a>
  to avoid throttling.
</Message>

A description second line has no replacement — fold it into the single copy line. The __title / __description testids are replaced by one __content testid.

Notes

• Sizes. small — 32px floor, 8px inline padding, .text-label-sm; medium (default) — 36px, 12px, .text-label-md. Block padding is 6px in both. A trailing action or close control tightens the end edge to 8px and lets the banner grow past the floor.
• Trailing controls track size. The built-in action is now a secondary Button (was text) so it reads as a control rather than body copy; both it and the close IconButton take small (28px) / medium (32px) with the banner.
• Leading icon is a fixed 14px (size-3.5 box, .text-label-md glyph). It previously used text-[length:inherit], which made the glyph inherit the host page's font-size rather than a token.
• Call sites updated: the DeleteDomainDialog template story, and the title → label usages in the shipped webkit-form, webkit-ui-states, and webkit-ui-verify skills. Those skill edits ride along because they would otherwise document a prop that no longer exists.
• Tests: 33 assertions in Vitest browser mode (Chromium) cover the content region, the size mapping, the data-trailing flag, severity/role normalization, the action and close controls, and axe. All pass.
• Visual baselines are regenerated in a follow-up commit on this branch — the 24 existing Message snapshots are stale, and the new Sizes / WithLinks stories need new ones.

Known gaps, not addressed here

• The action/close sizes are type-checked but not behaviourally tested: Button and IconButton expose no data-size, and .claude/rules/testing.md forbids class-string assertions. Adding data-size to those two legacy components is the real fix.
• The spec's a11y section claims "touch target ≥40×40 px"; the trailing controls are 28/32px. Pre-existing (the close was already 28px), and it clears WCAG 2.2's 24px AA minimum, but the claim and the implementation should be reconciled — a design call.
• On size="small" the 12px gap now exceeds the 8px inline padding, so the icon sits closer to the banner edge than to the copy. Left as-is; data-[size=small]:gap-[var(--spacing-xs)] would even it out.