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/entities/system/system_test.go

@@ -0,0 +1,37 @@
+package system
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/fxamacker/cbor/v2"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStatsBatteryTransport(t *testing.T) {
+	stats := Stats{Battery: [2]uint8{0, 1}, Batteries: map[string]uint8{"Primary": 0, "Mouse": 75}}
+
+	jsonData, err := json.Marshal(stats)
+	require.NoError(t, err)
+	var jsonPayload map[string]any
+	require.NoError(t, json.Unmarshal(jsonData, &jsonPayload))
+	assert.Equal(t, []any{float64(0), float64(1)}, jsonPayload["bat"])
+	assert.Equal(t, map[string]any{"Primary": float64(0), "Mouse": float64(75)}, jsonPayload["bats"])
+
+	cborData, err := cbor.Marshal(stats)
+	require.NoError(t, err)
+	var decoded Stats
+	require.NoError(t, cbor.Unmarshal(cborData, &decoded))
+	assert.Equal(t, stats.Battery, decoded.Battery)
+	assert.Equal(t, stats.Batteries, decoded.Batteries)
+}
+
+func TestStatsLegacyBatteryPayload(t *testing.T) {
+	data, err := json.Marshal(Stats{Battery: [2]uint8{50, 4}})
+	require.NoError(t, err)
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal(data, &payload))
+	assert.Contains(t, payload, "bat")
+	assert.NotContains(t, payload, "bats")
+}