Add request comparison feature to web UI

- Add Compare button in header to enter compare mode
- Allow selecting 2 requests via checkboxes for side-by-side comparison
- Create RequestCompareModal component with:
  - Summary stats (added/removed/modified/unchanged messages)
  - Side-by-side request metadata comparison
  - Message diff view with color-coded changes
  - System prompt comparison
  - Tools comparison (added/removed/common)
- Sticky compare mode banner that persists while scrolling
- Button label changes based on state (Compare / Exit Compare)

Author: ejc3