feat(stat-card): add copy-to-clipboard affordance on Dashboard stat values (Closes #770)

[waterWang]

Summary

Adds a hover-reveal copy-to-clipboard button on each stat value in the Dashboard's StatCard component (success state). When clicked, the stat value is copied to the clipboard and a success toast is shown.

Changes

• components/cards/stat-card.tsx:

  • Added a Copy icon button that appears on hover (opacity-0 → opacity-100 on group-hover)
  • Button is visible on focus-visible for keyboard users (focus-visible:opacity-100)
  • Uses the existing useToast hook for success/error feedback
  • Transitions the icon from Copy → Check (green) for 2 seconds after a successful copy
  • Includes a sr-only live region for screen-reader announcements
  • Cleans up timeouts on unmount

• components/cards/__tests__/stat-card.copy.test.tsx (new):

  • 5 focused tests for the copy-to-clipboard behavior
  • Tests: renders copy button, not shown in empty/loading/error, copied state, keyboard accessibility, clipboard unavailable

Accessibility

• The copy button is a native <button> element, keyboard-reachable and focusable
• Uses aria-label for descriptive copy action text
• Uses aria-pressed to convey the transient copied state
• Has a sr-only aria-live=polite region for screen-reader announcements
• Hover-reveal pattern with focus-visible:opacity-100 ensures keyboard users can see the button

Closes #770

[vercel]

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