bug: Prevent compare swap from refetching user data #189

Clicking the swap button on the compare page should only swap the two already-loaded users in the UI and update the URL order. Currently it can trigger a fresh /api/compare request after the URL changes, even though the same two users are already available locally.

Context

• components/compare-form.tsx correctly uses type="button" for the swap action, so this is not a form-submit issue.
• components/home-page-client.tsx swaps username1/username2, calls router.push(...), and then swaps data.user1/data.user2 locally.
• The useEffect that watches searchParams runs after the router.push and calls syncToUrl(...).
• syncToUrl(...) builds a new fetch key with the reversed username order, so it does not match lastFetchedKeyRef.current and calls handleCompare(..., { updateUrl: false }), causing an unnecessary refetch.

Tasks

• Make compare fetch deduplication treat swapped username pairs as the same comparison when selected languages are unchanged, or track a swap-only URL update so syncToUrl does not refetch.
• Ensure swapUsers() preserves selected language query params when updating the URL.
• Keep the local data swap behavior so the UI updates immediately without loading state.
• Add a regression test or interaction test that verifies clicking swap does not call /api/compare again.