Purpose

Search queries, filters, and sorting were stored in local React state, so users lost their selected settings when refreshing the page or navigating away, and could not share filtered results. This PR stores them in URL query parameters.

Changes

• Add useCatalogParams hook (frontend/src/hooks/useCatalogParams.ts) that reads and writes search, category, sortBy and sortDirection in the URL. Invalid values are ignored.
• Remove the shared search state and the key={search} workaround from App.tsx; the header search now navigates to /explore?search=…, so NavBar no longer takes an onSearch prop.
• Typing in the search field replaces the current history entry (no entry per keystroke); changing filters or sorting adds one, so back/forward restores them. Reset removes the category and sort parameters and keeps the search.
• Add tests in frontend/src/test/useCatalogParams.test.tsx.

Related issues

Closes #94

Verification

• `npx vitest run: 31 tests pass (6 new).

Manual testing:

1. On /explore, set a search, category and sort. The URL updates.
2. Refresh, or open the URL in a new tab; the same results are shown.
3. Change filters, then use browser Back/Forward; the filters and results follow.

Checklist

• I have reviewed my own changes.
• The changes meet the linked issue's acceptance criteria, where applicable.
• I have verified the changes and recorded the results above.
• I have added or updated tests where needed.
• I have updated documentation where needed.
• I have included screenshots for visual changes, or marked them not applicable.