feat: Denkovi USB 4/8 (selectable) + generic USB-serial relay board (CH340/LCUS)

- Denkovi (FT245 D2XX) now selectable as 4 or 8 relays (a 4-relay board just uses
  the low nibble); NewDenkovi takes a count, driven by the device's Channels.
- New 'usbrelay' backend: cheap CH340/LCUS USB-serial boards over a COM port
  using the common A0 protocol ([0xA0][relay][state][checksum]); write-only, so
  Status is a shadow. Channel count configurable (1/2/4/8/16).
- StationDevice gains Channels; deviceRelayCount() drives label/relay counts for
  the variable-size types. Device editor: channel selector + COM-port picker
  (usbrelay) / FTDI-serial (denkovi).

Both untested on hardware; the A0 command set / Denkovi bit order may need a tweak
after a live test.

app.go

@@ -11397,14 +11397,28 @@ func boolStr(b bool) string {
 // StationDevice is one configured relay board for the Station Control tab.
 type StationDevice struct {
 	ID     string   `json:"id"`
-	Type   string   `json:"type"` // "webswitch" | "kmtronic" | "denkovi"
+	Type   string   `json:"type"` // "webswitch" | "kmtronic" | "denkovi" | "usbrelay"
 	Name   string   `json:"name"`
-	Host   string   `json:"host"`
+	Host   string   `json:"host"` // IP for network boards; FTDI serial for denkovi; COM port for usbrelay
 	User   string   `json:"user,omitempty"` // KMTronic HTTP auth (blank = none)
 	Pass   string   `json:"pass,omitempty"`
+	Channels int    `json:"channels,omitempty"` // usbrelay only: number of relays (1/2/4/8/16)
 	Labels []string `json:"labels"` // per-relay label (index 0 = relay 1)
 }
 
+// deviceRelayCount is the relay count for a configured device — fixed by type,
+// except the Denkovi (4 or 8) and the generic USB-serial board, whose channel
+// count the user picks.
+func deviceRelayCount(d StationDevice) int {
+	if d.Type == "usbrelay" || d.Type == "denkovi" {
+		if d.Channels >= 1 {
+			return d.Channels
+		}
+		return 8
+	}
+	return relayCountFor(d.Type)
+}
+
 // relayCountFor returns a device type's fixed relay count.
 func relayCountFor(typ string) int {
 	switch typ {
@@ -11422,7 +11436,10 @@ func deviceDriver(d StationDevice) relaydev.Device {
 		return relaydev.NewKMTronic(d.Host, d.User, d.Pass)
 	case "denkovi":
 		// Host carries the FTDI serial number (e.g. "DAE0006K"), not a hostname.
-		return relaydev.NewDenkovi(d.Host)
+		return relaydev.NewDenkovi(d.Host, deviceRelayCount(d))
+	case "usbrelay":
+		// Host carries the COM port (e.g. "COM5"); CH340/LCUS "A0" serial protocol.
+		return relaydev.NewSerialRelay(d.Host, deviceRelayCount(d))
 	default:
 		return relaydev.NewWebswitch(d.Host)
 	}
@@ -11458,7 +11475,7 @@ func (a *App) SaveStationDevices(devs []StationDevice) error {
 	}
 	for i := range devs {
 		switch devs[i].Type {
-		case "kmtronic", "denkovi", "webswitch":
+		case "kmtronic", "denkovi", "usbrelay", "webswitch":
 			// known type — keep
 		default:
 			devs[i].Type = "webswitch"
@@ -11466,7 +11483,7 @@ func (a *App) SaveStationDevices(devs []StationDevice) error {
 		if strings.TrimSpace(devs[i].ID) == "" {
 			devs[i].ID = fmt.Sprintf("dev%d-%d", i, time.Now().UnixNano())
 		}
-		n := relayCountFor(devs[i].Type)
+		n := deviceRelayCount(devs[i])
 		for len(devs[i].Labels) < n {
 			devs[i].Labels = append(devs[i].Labels, "")
 		}
@@ -11507,7 +11524,7 @@ func (a *App) GetStationStatus() []StationDeviceStatus {
 		go func(i int) {
 			defer wg.Done()
 			d := devs[i]
-			n := relayCountFor(d.Type)
+			n := deviceRelayCount(d)
 			ds := StationDeviceStatus{ID: d.ID, Name: d.Name, Type: d.Type}
 			ctx, cancel := context.WithTimeout(a.ctx, 6*time.Second)
 			defer cancel()