Add LED ring control per client and broadcast over REST and WebSocket.

Solid color mode fills all ring LEDs; master routes UART commands to slaves
via ESPNOW_LED_RING. goTool exposes POST /api/led-ring, WebSocket set_led_ring,
and a dashboard LED panel with master/slave/all targets.

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

goTool/api_led_ring.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+func mountLedRingAPI(mux *http.ServeMux, link *managedSerial) {
+	mux.HandleFunc("/api/led-ring", func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+			return
+		}
+		serveLedRingPost(w, r, link)
+	})
+}
+
+func serveLedRingPost(w http.ResponseWriter, r *http.Request, link *managedSerial) {
+	var body ledRingAPIRequest
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeJSON(w, http.StatusBadRequest, ledRingAPIResponse{Error: "invalid JSON"})
+		return
+	}
+	if body.Mode == "" {
+		writeJSON(w, http.StatusBadRequest, ledRingAPIResponse{Error: "mode required"})
+		return
+	}
+	out := applyLedRing(link, body)
+	status := http.StatusOK
+	if out.Error != "" {
+		status = http.StatusServiceUnavailable
+	} else if !out.Success {
+		status = http.StatusServiceUnavailable
+	}
+	writeJSON(w, status, out)
+}