chore: TUI screenshots, README polish, changelog rewrite (#32)

- Add 6 TUI screenshots to assets/ (monitors, alerts, logs, nodes, detail, theme)
- Rewrite README with hero image, badges, collapsible install sections
- Rewrite changelog to match actual CalVer tag history
- VHS tooling extracted to lerko/uptop-vhs

Reviewed-on: lerko/uptop#32

internal/store/store.go

@@ -49,6 +49,10 @@ type Store interface {
 	UpdateNodeLastSeen(id string) error
 	DeleteNode(id string) error
 
+	// Alert Health
+	LoadAlertHealth() (map[int]models.AlertHealthRecord, error)
+	SaveAlertHealth(h models.AlertHealthRecord) error
+
 	// Logs
 	SaveLog(message string) error
 	LoadLogs(limit int) ([]string, error)