test: add useDebounce integration tests and state management docs (#489 #490 #496)

[Chidimj]

Summary

• Add integration test for useDebounce hook not updating value before delay elapses #496 - Integration test for useDebounce hook: asserts debounced value stays unchanged during the delay window and updates after timer advances past it (fake timers, deterministic)
• Add integration test for debounced search input only triggering one API call after rapid typing #489 - Integration test for debounced creator search: asserts API is called exactly once after rapid typing burst and uses the final input value, with no calls fired mid-burst
• Add docs for client state management approach and when to use local vs server state #490 - Docs for client state management: documents the server-state (React Query) vs ephemeral UI state (useState) rule, query invalidation vs manual refetch guidance, and an anti-pattern example

Test plan

• pnpm test src/hooks/tests/useDebounce.test.tsx -- 3 tests pass
• pnpm test src/pages/tests/LandingPage.debouncedSearch.integration.test.tsx -- 3 tests pass
• docs/state-management.md reviewed for accuracy against codebase patterns

Closes #489
Closes #490
Closes #496