feat: hide empty sessions and auto-refresh message drawer

[Oaklight]

Summary

• Admin panel: hide or remove unused empty sessions #22: Filter out sessions that have never been used (no bot_id, not connected, no users) from the session list — eliminates the phantom "default" session clutter
• Admin panel message drawer should auto-refresh #24: Auto-poll for new messages every 5s while the message drawer is open, appending new bubbles at the bottom with auto-scroll when already at bottom

Closes #22
Closes #24