feat(keys): supply cap progress indicator on key detail page

[chidii]

Closes #1035

Summary

Adds a SupplyCapProgress component and wires it into the key detail page,
showing current supply as a percentage of the cap with a sold-out state
when the cap is reached.

Changes

• src/components/common/SupplyCapProgress.tsx — progress bar with numeric
  current / cap / remaining display, sold-out badge, and full hide when the
  cap is null / undefined / 0
• src/components/common/__tests__/SupplyCapProgress.test.tsx — 9 tests
  covering every acceptance criterion
• src/services/course.service.ts — add supplyCap?: number | null to
  KeyStats so the API field is typed and ready
• src/pages/CreatorDetailPage.tsx — render <SupplyCapProgress> below
  the Buy button and disable Buy when supply >= supplyCap

Acceptance criteria

• Progress bar shows correct current/cap ratio
• Numeric values display correctly at all supply levels
• Sold-out badge appears when supply equals cap
• Buy button correctly disabled in sold-out state
• Progress bar not rendered for unlimited keys

Notes

• The /keys/:id/stats endpoint does not currently return supplyCap, so the
  component renders nothing for every key today. Once the API includes
  supplyCap, the bar activates automatically — no further client changes
  needed.
• The 30s refresh is inherited from useKeyStats, whose refetchInterval
  is already 30_000 (KEY_STATS_REFRESH_INTERVAL_MS), so no new polling
  was added.

Verification

• pnpm run lint — 0 problems
• pnpm run build — ✓ built in 43.86s
• pnpm run test -- SupplyCapProgress — 9 passed (9)

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