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/postgres.go

@@ -67,6 +67,10 @@ func (d *PostgresDialect) CreateTablesSQL() []string {
 			created_by TEXT DEFAULT '',
 			created_at TIMESTAMP DEFAULT NOW()
 		)`,
+		`CREATE TABLE IF NOT EXISTS preferences (
+			key TEXT PRIMARY KEY,
+			value TEXT NOT NULL
+		)`,
 	}
 }