Add integration test for holder count display updating after React Qu…

[Johnpii1]

…ery cache invalidation

Summary

Description
Introduce a shared query key CREATOR_LIST_QUERY_KEY and move creator list fetching on the landing page to useQuery so query invalidation triggers refetches.
Surface live holder counts by adding holderCount?: number to the Course type and using featuredCreator.holderCount (with existing fallbacks) for the Audience chip.
Add an integration test that renders the landing page with a test QueryClient, mocks an initial holderCount, invalidates the creator query, mocks a refetch returning an updated holderCount, and asserts the UI shows the new count and the old count is gone.

Testing

Testing
Ran the focused integration test with pnpm exec vitest run src/pages/tests/LandingPage.keyboard.test.tsx and it passed.
Ran linting with pnpm exec eslint src/pages/LandingPage.tsx src/services/course.service.ts src/pages/tests/LandingPage.keyboard.test.tsx and it passed.
Ran type checks with pnpm exec tsc -b --pretty false and it passed.

• pnpm lint
• pnpm build

Checklist

• Linked issue or backlog item
• Scope is limited to the stated change
• Updated docs if behavior or setup changed
• Added screenshots for UI changes when relevant

close #480

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