ui: redesign BotSessionMonitor with polished chat UI

- Wider session list (w-72) with avatar circles and cleaner layout
- Richer chat header with avatar, platform info, and active indicator
- User messages now use blue-500 (solid) instead of blue-100 for
  clear visual distinction
- Metadata (time, runner) shown on hover below bubbles, not inside
- Proper empty state illustrations for both panels
- Better spacing, rounded corners, and shadow treatment
- Consistent dark mode styling

Author: RockChinQ