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/dashboard.go

@@ -19,7 +19,10 @@ type MasterView struct {
 	GitHash           string `json:"git_hash"`
 	RunningPartition  string `json:"running_partition,omitempty"`
 	Deadzone          uint32 `json:"deadzone,omitempty"`
-	OK                bool   `json:"ok"`
+	Lipo1             lipoReadingJSON `json:"lipo1"`
+	Lipo2             lipoReadingJSON `json:"lipo2"`
+	BatteryAgeMs      uint32          `json:"battery_age_ms,omitempty"`
+	OK                bool            `json:"ok"`
 	Error             string `json:"error,omitempty"`
 }
 
@@ -37,7 +40,10 @@ type ClientView struct {
 	AccelY          int32  `json:"accel_y"`
 	AccelZ          int32  `json:"accel_z"`
 	AccelAgeMs      uint32 `json:"accel_age_ms"`
-	AccelStream     bool   `json:"accel_stream"`
+	AccelStream     bool            `json:"accel_stream"`
+	Lipo1           lipoReadingJSON `json:"lipo1"`
+	Lipo2           lipoReadingJSON `json:"lipo2"`
+	BatteryAgeMs    uint32          `json:"battery_age_ms,omitempty"`
 }
 
 type DashboardState struct {
@@ -62,8 +68,16 @@ func newWSHub() *wsHub {
 
 func (h *wsHub) setState(st DashboardState) {
 	h.mu.Lock()
-	prev := h.state.Clients
-	st.Clients = preserveClientAccel(st.Clients, prev)
+	prev := h.state
+	st.Clients = preserveClientAccel(st.Clients, prev.Clients)
+	st.Clients = preserveClientBattery(st.Clients, prev.Clients)
+	if !st.Master.Lipo1.Valid && !st.Master.Lipo2.Valid {
+		if prev.Master.Lipo1.Valid || prev.Master.Lipo2.Valid {
+			st.Master.Lipo1 = prev.Master.Lipo1
+			st.Master.Lipo2 = prev.Master.Lipo2
+			st.Master.BatteryAgeMs = prev.Master.BatteryAgeMs
+		}
+	}
 	h.state = st
 	conns := make([]*websocket.Conn, 0, len(h.clients))
 	for c := range h.clients {
@@ -156,6 +170,33 @@ func preserveClientAccel(newClients, oldClients []ClientView) []ClientView {
 	return out
 }
 
+func preserveClientBattery(newClients, oldClients []ClientView) []ClientView {
+	if len(oldClients) == 0 {
+		return newClients
+	}
+	oldByID := make(map[uint32]ClientView, len(oldClients))
+	for _, c := range oldClients {
+		oldByID[c.ID] = c
+	}
+	out := make([]ClientView, len(newClients))
+	for i, c := range newClients {
+		out[i] = c
+		if c.Lipo1.Valid || c.Lipo2.Valid {
+			continue
+		}
+		prev, ok := oldByID[c.ID]
+		if !ok {
+			continue
+		}
+		if prev.Lipo1.Valid || prev.Lipo2.Valid {
+			out[i].Lipo1 = prev.Lipo1
+			out[i].Lipo2 = prev.Lipo2
+			out[i].BatteryAgeMs = prev.BatteryAgeMs
+		}
+	}
+	return out
+}
+
 func anyClientAccelStream(clients []ClientView) bool {
 	for _, c := range clients {
 		if c.AccelStream {
@@ -294,6 +335,7 @@ func pollDashboard(link *managedSerial, portName string, last *DashboardState, s
 		}
 		st.Clients = append(st.Clients, cv)
 	}
+	applyBatteryToState(link, &st)
 	if anyClientAccelStream(st.Clients) {
 		for i := range st.Clients {
 			if !st.Clients[i].AccelStream {
@@ -319,6 +361,60 @@ func pollDashboard(link *managedSerial, portName string, last *DashboardState, s
 	return st
 }
 
+func applyBatteryToState(link *managedSerial, st *DashboardState) {
+	bat, err := link.BatteryStatusPoll(&pb.BatteryStatusRequest{AllClients: true})
+	if err != nil {
+		log.Printf("battery poll: %v", err)
+		return
+	}
+	applyBatterySamplesToState(st, batterySamplesFromPB(bat.GetSamples()))
+}
+
+func (h *wsHub) mergeBattery(samples []batterySampleJSON) {
+	if len(samples) == 0 {
+		return
+	}
+	h.mu.Lock()
+	st := h.state
+	applyBatterySamplesToState(&st, samples)
+	st.UpdatedAt = time.Now().Format(time.RFC3339)
+	h.state = st
+	conns := make([]*websocket.Conn, 0, len(h.clients))
+	for c := range h.clients {
+		conns = append(conns, c)
+	}
+	h.mu.Unlock()
+
+	data, err := json.Marshal(st)
+	if err != nil {
+		return
+	}
+	for _, c := range conns {
+		_ = c.WriteMessage(websocket.TextMessage, data)
+	}
+}
+
+func runBatteryPoller(link *managedSerial, hub *wsHub, interval time.Duration, stop <-chan struct{}) {
+	ticker := time.NewTicker(interval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-stop:
+			return
+		case <-ticker.C:
+			if hub.clientCount() == 0 {
+				continue
+			}
+			bat, err := link.BatteryStatusPoll(&pb.BatteryStatusRequest{AllClients: true})
+			if err != nil {
+				continue
+			}
+			hub.mergeBattery(batterySamplesFromPB(bat.GetSamples()))
+		}
+	}
+}
+
 func runAccelDashboardPoller(link *managedSerial, hub *wsHub, interval time.Duration, stop <-chan struct{}) {
 	ticker := time.NewTicker(interval)
 	defer ticker.Stop()