Full-screen scrollable history view accessible via [h] from detail
panel. Shows all state transitions with computed outage durations,
event density sparkline for flapping detection, and summary stats.
- Detail panel STATE CHANGES now shows outage duration per recovery
- Event density sparkline highlights flapping periods
- Summary footer: event count, outage count, avg outage duration
- Vim-style navigation (j/k/g/G) + mouse scroll in history view
