design: undo banner pattern for reversible actions

[dumbdevss]

[UI/UX] Undo banner pattern for reversible destructive actions

Closes #162

What

An accessible Undo banner pattern for reversible destructive actions (delete
draft, remove from blacklist, archive offering).

Components

• UndoBanner — a stack of banners pinned above the page footer, each with a
  countdown ring to permanence, a primary Undo CTA, and a dismiss control
  that commits immediately. Newest on top; banners beyond maxVisible (default 3)
  collapse into a +N more pending summary. Responsive (w-full max-w-md,
  truncating message).
• useUndoBanners — owns the reversible-window timing via a single shared ticker;
  exposes registerUndo / undo / dismiss with onUndo and onCommit
  callbacks (the documented action contract).
• useReducedMotion — swaps the animated ring for a static seconds count under
  prefers-reduced-motion.

Accessibility (WCAG 2.1 AA)

• Polite live region (role="status", aria-live="polite").
• Decorative aria-hidden countdown; labelled controls with visible focus rings.
• Reduced-motion fallback; jest-axe asserts no violations.

Tests

• UndoBanner.test.tsx (10) + useUndoBanners.test.tsx (5) — 15 passed,
  lint clean.

Docs

docs/uiux/undo-banner-pattern.md — anatomy, engineer action contract,
stacking/placement, responsive, and accessibility/axe notes.

Notes

• Added jest-axe as a devDependency for the accessibility assertion.

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