feat: add multi-battery monitoring

- Report battery data for individual devices
- Select a representative battery for legacy fields and alerts
- Average named battery data independently
- Display multiple batteries in system charts
- Add cross-platform coverage and transport tests

internal/alerts/alerts.go

@@ -55,6 +55,7 @@ type SystemAlertStats struct {
 	Temperatures map[string]float32            `json:"t"`
 	LoadAvg      [3]float64                    `json:"la"`
 	Battery      [2]uint8                      `json:"bat"`
+	Batteries    map[string]uint8              `json:"bats"`
 	ExtraFs      map[string]SystemAlertFsStats `json:"efs"`
 }