perf: implement virtual scrolling for long lists (leaderboard, comments, proposals)

Long lists can degrade rendering performance on lower-end devices.\n\n**Action:**\n- [ ] Identify lists that can exceed 100 items in production\n- [ ] Implement virtual scrolling for leaderboard (react-virtual or similar)\n- [ ] Implement virtual scrolling for proposal list\n- [ ] Implement virtual scrolling for comment sections on popular proposals\n- [ ] Measure frame rate improvement on mobile