perf(frontend): add client-side query cache for API data

[Flickwire-Agent]

Summary

Add a small query cache/SWR layer for repeated page data fetches.

Evidence

• Pages manage fetch state locally: apps/web/src/components/OpportunitiesPage.tsx:57, apps/web/src/components/BargainsPage.tsx:27, apps/web/src/components/ListingsPage.tsx:32, and apps/web/src/components/ItemPage.tsx:27.
• Backend cache cadence is usually 15 minutes.

Expected Impact

Medium. Improves back/forward navigation and reduces duplicate network requests.

Implementation Notes

Use a lightweight custom cache or a library. Set stale time to match backend refresh for cache-backed endpoints.

Acceptance Criteria

• Route revisits can show cached data immediately.
• Stale data refreshes in the background where appropriate.