#500: feat(performance): virtual scroll for session list and message list

Summary

Users with large session histories or very long conversations experience slow rendering and DOM bloat. Both the session sidebar and the message list currently render all items into the DOM at once.

Proposed behaviour

Implement virtual (windowed) scrolling so only the items near the current scroll position are in the DOM:

1. Session sidebar — ~20 visible items plus a small buffer
2. Message list — messages near the current scroll position

Implementation notes

• Use IntersectionObserver for a lightweight no-library approach: replace off-screen items with placeholder divs of the same height
• No bundler or external dependency needed — keeps the project's no-build-step constraint
• Session list is simpler (roughly fixed height per item); message list is harder (variable-height markdown)
• Suggested order: session list first, message list as a follow-on

Tracked in ROADMAP.md as "Virtual scroll for large lists (deferred)".