Add goTool web dashboard with UART auto-reconnect.

Serve polls the master over UART and pushes live state via WebSocket;
reopens the serial port when the device is unplugged and comes back.

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

goTool/cmd_serve.go

@@ -0,0 +1,69 @@
+package main
+
+import (
+	"embed"
+	"flag"
+	"fmt"
+	"io/fs"
+	"log"
+	"net/http"
+	"time"
+
+	"github.com/gorilla/websocket"
+)
+
+//go:embed webui/*
+var webUI embed.FS
+
+var wsUpgrader = websocket.Upgrader{
+	CheckOrigin: func(r *http.Request) bool { return true },
+}
+
+func runServe(portName string, baud int, args []string) error {
+	serveFlags := flag.NewFlagSet("serve", flag.ExitOnError)
+	addr := serveFlags.String("addr", ":8080", "HTTP listen address")
+	interval := serveFlags.Duration("interval", 2*time.Second, "UART poll interval")
+	if err := serveFlags.Parse(args); err != nil {
+		return err
+	}
+	if portName == "" {
+		return fmt.Errorf("serve requires -port (master UART)")
+	}
+
+	link := newManagedSerial(portName, baud)
+	link.quiet = true
+	defer link.Close()
+
+	hub := newWSHub()
+	stop := make(chan struct{})
+	defer close(stop)
+	go runPoller(link, portName, hub, *interval, stop)
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/ws", func(w http.ResponseWriter, r *http.Request) {
+		conn, err := wsUpgrader.Upgrade(w, r, nil)
+		if err != nil {
+			log.Printf("websocket upgrade: %v", err)
+			return
+		}
+		hub.register(conn)
+		defer hub.unregister(conn)
+		defer conn.Close()
+
+		for {
+			if _, _, err := conn.ReadMessage(); err != nil {
+				return
+			}
+		}
+	})
+
+	ui, err := fs.Sub(webUI, "webui")
+	if err != nil {
+		return err
+	}
+	mux.Handle("/", http.FileServer(http.FS(ui)))
+
+	log.Printf("dashboard http://localhost%s  (UART %s @ %d baud, poll %s, auto-reconnect)",
+		*addr, portName, baud, interval.String())
+	return http.ListenAndServe(*addr, mux)
+}