Stellar Wave: key price display, card a11y, sort animation, drop countdown

[codellins]

Summary

• Add a shared stroops-to-XLM formatter with stroops fallback and use it on creator cards and the buy confirmation dialog (Add helper for displaying creator key price in XLM with stroops fallback #353).
• Label the creator card purchase action group with an sr-only heading for screen reader context (Add sr-only label for creator card action button group #354).
• Animate marketplace card reordering when sort changes, respecting prefers-reduced-motion (Add transition animation for marketplace sort order change #355).
• Add a live drop countdown that updates every second and switches to a drop-live message at zero (Add helper for displaying time remaining until next creator drop #362).

Verification

• pnpm exec tsc -b --noEmit passes.
• pnpm test src/utils/__tests__/keyPriceDisplay.utils.test.ts src/utils/__tests__/dropCountdown.utils.test.ts passes (9 tests).
• Manually change marketplace sort and confirm cards reorder with a brief spring transition; enable reduced motion in OS settings and confirm reorder is instant.
• Open a creator card with nextDropAt demo data and confirm the countdown ticks each second until it shows “Drop live”.
• With a screen reader, focus the creator card buy row and confirm the group label is announced before the buy button.

Closes #353
Closes #354
Closes #355
Closes #362

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