feat: add unique trader count stat to key detail analytics (#1020)

[daniella-techie]

Closes #1020
Closes #1019
Closes #1021
Closes #1022

Summary

Adds a Unique Traders stat card to the Key Stats panel on the creator key detail page, alongside holders and total volume.

Changes

• courseService.getKeyUniqueTraders(keyId) → GET /keys/:keyId/unique-traders, returning { uniqueTraders, uniqueTraders24hAgo }
• useKeyUniqueTraders hook, refreshed every 5 minutes (staleTime and refetchInterval)
• UniqueTradersStat cell rendered inside KeyStatsPanel:
  • Fixed-height value slot, with the skeleton shown only on the first load, so refreshes don't shift the layout
  • Accessible tooltip: "Number of distinct wallets that have bought or sold this key at least once."
  • 24h trend indicator (up/down/no change) with a screen-reader label; hidden when the previous value is unavailable
  • New keys with zero traders show 0 with a no-change trend
• Panel grid updated to sm:grid-cols-4 xl:grid-cols-7 to fit the seventh cell
• There is no key summary export in the codebase yet, so the "include in export" item is not applicable

Tests

• UniqueTradersStat.test.tsx: value, up/down/flat trend, zero traders, missing previous value, tooltip, no layout shift, background refresh
• useKeyUniqueTraders.test.tsx: 5-minute refresh config, refetch keeps previous data, disabled for an empty id
• uniqueTraders.utils.test.ts: trend calculation
• KeyStatsPanel.test.tsx updated for the new cell and grid

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