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/api_battery.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+	"strconv"
+)
+
+func mountBatteryAPI(mux *http.ServeMux, link *managedSerial) {
+	mux.HandleFunc("/api/battery", func(w http.ResponseWriter, r *http.Request) {
+		switch r.Method {
+		case http.MethodGet:
+			serveBatteryGet(w, r, link)
+		case http.MethodPost:
+			serveBatteryPost(w, r, link)
+		default:
+			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		}
+	})
+}
+
+func serveBatteryGet(w http.ResponseWriter, r *http.Request, link *managedSerial) {
+	req := batteryAPIRequest{}
+	if v := r.URL.Query().Get("all_clients"); v == "1" || v == "true" {
+		req.AllClients = true
+	}
+	if s := r.URL.Query().Get("client_id"); s != "" {
+		id, err := strconv.ParseUint(s, 10, 32)
+		if err != nil {
+			writeJSON(w, http.StatusBadRequest, batteryAPIResponse{Error: "invalid client_id"})
+			return
+		}
+		req.ClientID = uint32(id)
+	} else if !req.AllClients {
+		req.AllClients = true
+	}
+	out := applyBatteryStatus(link, req)
+	status := http.StatusOK
+	if out.Error != "" || !out.Success {
+		status = http.StatusServiceUnavailable
+	}
+	writeJSON(w, status, out)
+}
+
+func serveBatteryPost(w http.ResponseWriter, r *http.Request, link *managedSerial) {
+	var body batteryAPIRequest
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeJSON(w, http.StatusBadRequest, batteryAPIResponse{Error: "invalid JSON"})
+		return
+	}
+	if !body.AllClients && body.ClientID == 0 {
+		body.AllClients = true
+	}
+	out := applyBatteryStatus(link, body)
+	status := http.StatusOK
+	if out.Error != "" || !out.Success {
+		status = http.StatusServiceUnavailable
+	}
+	writeJSON(w, status, out)
+}