Add LiPo battery monitoring with ESP-NOW cache and dashboard API.

Slaves report pack voltages every 30s; the master caches them for fast
BATTERY_STATUS reads. goTool exposes REST/WebSocket and shows values in
the dashboard, with a nanopb fix so optional lipo submessages encode.

Co-authored-by: Cursor <cursoragent@cursor.com>

goTool/battery_api.go

@@ -0,0 +1,120 @@
+package main
+
+import (
+	"powerpod/gotool/pb"
+)
+
+const (
+	lipoMinMv = 3000
+	lipoMaxMv = 4200
+)
+
+type lipoReadingJSON struct {
+	Valid     bool   `json:"valid"`
+	VoltageMv uint32 `json:"voltage_mv"`
+	Percent   int    `json:"percent,omitempty"`
+}
+
+type batterySampleJSON struct {
+	ClientID uint32          `json:"client_id"`
+	Lipo1    lipoReadingJSON `json:"lipo1"`
+	Lipo2    lipoReadingJSON `json:"lipo2"`
+	AgeMs    uint32          `json:"age_ms,omitempty"`
+}
+
+type batteryAPIRequest struct {
+	ClientID   uint32 `json:"client_id"`
+	AllClients bool   `json:"all_clients"`
+}
+
+type batteryAPIResponse struct {
+	Type     string              `json:"type,omitempty"` // battery_status (WebSocket)
+	Success  bool                `json:"success"`
+	Samples  []batterySampleJSON `json:"samples,omitempty"`
+	Error    string              `json:"error,omitempty"`
+}
+
+func lipoPercent(mv uint32) int {
+	if mv <= lipoMinMv {
+		return 0
+	}
+	if mv >= lipoMaxMv {
+		return 100
+	}
+	return int((mv - lipoMinMv) * 100 / (lipoMaxMv - lipoMinMv))
+}
+
+func lipoFromPBMsg(l *pb.LipoReading) lipoReadingJSON {
+	if l == nil {
+		return lipoReadingJSON{}
+	}
+	return lipoFromPB(l.GetValid(), l.GetVoltageMv())
+}
+
+func lipoFromPB(valid bool, mv uint32) lipoReadingJSON {
+	out := lipoReadingJSON{Valid: valid, VoltageMv: mv}
+	if valid {
+		out.Percent = lipoPercent(mv)
+	}
+	return out
+}
+
+func batterySamplesFromPB(samples []*pb.BatterySample) []batterySampleJSON {
+	out := make([]batterySampleJSON, 0, len(samples))
+	for _, s := range samples {
+		out = append(out, batterySampleJSON{
+			ClientID: s.GetClientId(),
+			Lipo1:    lipoFromPBMsg(s.GetLipo1()),
+			Lipo2:    lipoFromPBMsg(s.GetLipo2()),
+			AgeMs:    s.GetAgeMs(),
+		})
+	}
+	return out
+}
+
+func applyBatteryStatus(link *managedSerial, in batteryAPIRequest) batteryAPIResponse {
+	resp, err := link.BatteryStatus(&pb.BatteryStatusRequest{
+		ClientId:   in.ClientID,
+		AllClients: in.AllClients,
+	})
+	if err != nil {
+		return batteryAPIResponse{Error: err.Error()}
+	}
+	samples := batterySamplesFromPB(resp.GetSamples())
+	out := batteryAPIResponse{
+		Success: resp.GetSuccess() || len(samples) > 0,
+		Samples: samples,
+	}
+	if len(samples) == 0 && out.Error == "" {
+		out.Error = "battery status unavailable"
+	}
+	return out
+}
+
+func findBatterySample(samples []batterySampleJSON, clientID uint32) (batterySampleJSON, bool) {
+	for _, s := range samples {
+		if s.ClientID == clientID {
+			return s, true
+		}
+	}
+	return batterySampleJSON{}, false
+}
+
+// applyBatterySamplesToState merges UART/REST battery samples into dashboard views.
+func applyBatterySamplesToState(st *DashboardState, samples []batterySampleJSON) {
+	if st == nil || len(samples) == 0 {
+		return
+	}
+	if m, ok := findBatterySample(samples, 0); ok {
+		st.Master.Lipo1 = m.Lipo1
+		st.Master.Lipo2 = m.Lipo2
+		st.Master.BatteryAgeMs = m.AgeMs
+	}
+	for i := range st.Clients {
+		if s, ok := findBatterySample(samples, st.Clients[i].ID); ok {
+			st.Clients[i].Lipo1 = s.Lipo1
+			st.Clients[i].Lipo2 = s.Lipo2
+			st.Clients[i].BatteryAgeMs = s.AgeMs
+		}
+	}
+}