feat(tui): zebra striping, detail breadcrumb, sparkline stats, collapse persistence

Add alternating row backgrounds for easier table scanning. Detail panel
now shows breadcrumb path (Sites > Group > Name) and min/avg/max latency
stats below the sparkline. Group collapse state persists across restarts
via new preferences table in both SQLite and Postgres.

internal/store/store.go

@@ -57,6 +57,10 @@ type Store interface {
 	DeleteMaintenanceWindow(id int) error
 	IsMonitorInMaintenance(monitorID int) (bool, error)
 
+	// Preferences
+	GetPreference(key string) (string, error)
+	SetPreference(key, value string) error
+
 	// Backup & Restore
 	ExportData() (models.Backup, error)
 	ImportData(data models.Backup) error