feat: infinite scroll for past parties in registration tracker

[VidurShah]

The registration tracker rendered all past party cards at once with no limit, which could cause jank on mobile for users with many parties.

Changes:

• Added useInfiniteScroll hook — wraps IntersectionObserver to progressively reveal items from an in-memory list as the user scrolls to a sentinel element. Uses refs for total/pageSize inside the callback to avoid stale closures, and only resets the visible count when the list shrinks (not on every data refresh).
• Replaced the unbounded pastParties.map(...) in RegistrationTracker with visiblePastParties (sliced to visibleCount) and a sentinel <div> that triggers the next page load when it enters the viewport.
• Kept active parties and incidents unbounded — active parties are expected to be few, and incidents don't benefit from the same pattern.
• No backend pagination needed: researched the data volume question from the issue. A student's lifetime party count is unlikely to exceed a few dozen, so client-side slicing is sufficient. The hook is still fully decoupled and could be swapped for a server-paginated version later if needed.

Closes #438

[github-actions]

Test Results Summary

870 tests  ±0   870 ✅ ±0   1m 9s ⏱️ ±0s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 373bad4. ± Comparison against base commit 2004c9c.

♻️ This comment has been updated with latest results.

[naasanov]

lgtm