feat: add useDebounce hook, integration tests, and state management docs

[Chidimj]

Summary

• Add integration test for activity feed showing correct sign prefix on buy and sell amounts #482 – Fixed TransactionHistory (activity feed) to show buy amounts with - prefix (XLM spent) and sell amounts with + prefix (XLM received); added XLM suffix throughout. Integration test asserts both sign conventions and the XLM suffix.
• Add integration test for useDebounce hook not updating value before delay elapses #496 – Created src/hooks/useDebounce.ts and an integration test confirming the debounced value does not update before the delay elapses, and does update after it.
• Add integration test for debounced search input only triggering one API call after rapid typing #489 – Wired the debounced search query to courseService.getCourses() in LandingPage via the new useDebounce hook. Integration test (minimal harness) confirms: no API calls during the typing burst, exactly one call fires after the delay with the final value, and an empty query omits the search param.
• Add docs for client state management approach and when to use local vs server state #490 – Added docs/state-management.md documenting the server-vs-local state rule, query invalidation vs manual refetch guidance, and an anti-pattern example.

Test plan

• pnpm test -- --run src/components/common/__tests__/TransactionHistory.test.tsx — 3 tests pass (sign prefix + XLM suffix)
• pnpm test -- --run src/hooks/__tests__/useDebounce.test.tsx — 3 tests pass (debounce timing)
• pnpm test -- --run src/pages/__tests__/LandingPage.debouncedSearch.integration.test.tsx — 3 tests pass (API wiring + suppression + clear)
• Review docs/state-management.md for accuracy

Closes #482, #489, #490, #496

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