feat: add user list pagination

UserList now uses useInfiniteQuery with offset pagination (20 per page)
and an IntersectionObserver scroll sentinel for infinite scroll.
Users sorted by username. Follows same pattern as Feed/UserFeed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/js/constants.ts

@@ -1,2 +1,3 @@
 export const FEED_PAGE_SIZE = 10;
 export const COMMENTS_PAGE_SIZE = 10;
+export const USERS_PAGE_SIZE = 20;