feat: Denkovi USB 8-relay board (FT245 D2XX bit-bang)

Third relay device alongside WebSwitch and KMTronic. Despite enumerating as a
COM port, this board is driven via FTDI D2XX synchronous bit-bang (one byte = 8
relays, bit 0 = relay 1), not serial ASCII — so we load ftd2xx.dll at runtime
(no CGO) and call FT_OpenEx/FT_SetBitMode/FT_Write, addressing the board by its
FTDI serial (e.g. DAE0006K), exactly like the vendor tool. Windows-only (stub
elsewhere); ListDenkoviDevices enumerates connected serials for the picker.

Wired into relaydev (Count/Status/Set), app.go (deviceDriver/relayCountFor/type
whitelist + ListDenkoviDevices binding), and the Station device editor (new type
+ FTDI-serial field with Detect). Untested on hardware; bit order / on-polarity
may need a flip after a live test.

app.go

@@ -11397,7 +11397,7 @@ 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"
+	Type   string   `json:"type"` // "webswitch" | "kmtronic" | "denkovi"
 	Name   string   `json:"name"`
 	Host   string   `json:"host"`
 	User   string   `json:"user,omitempty"` // KMTronic HTTP auth (blank = none)
@@ -11407,18 +11407,31 @@ type StationDevice struct {
 
 // relayCountFor returns a device type's fixed relay count.
 func relayCountFor(typ string) int {
-	if typ == "kmtronic" {
+	switch typ {
+	case "kmtronic", "denkovi":
 		return 8
+	default:
+		return 5 // webswitch 1216H
 	}
-	return 5 // webswitch 1216H
 }
 
 // deviceDriver builds the wire driver for a configured device.
 func deviceDriver(d StationDevice) relaydev.Device {
-	if d.Type == "kmtronic" {
+	switch d.Type {
+	case "kmtronic":
 		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)
+	default:
+		return relaydev.NewWebswitch(d.Host)
 	}
-	return relaydev.NewWebswitch(d.Host)
+}
+
+// ListDenkoviDevices returns the FTDI serial numbers of connected Denkovi/FTDI
+// boards, for the settings picker. Windows-only (FTDI D2XX).
+func (a *App) ListDenkoviDevices() ([]string, error) {
+	return relaydev.ListDenkovi()
 }
 
 // GetStationDevices returns the configured relay boards (without live state).
@@ -11444,7 +11457,10 @@ func (a *App) SaveStationDevices(devs []StationDevice) error {
 		return fmt.Errorf("db not initialized")
 	}
 	for i := range devs {
-		if devs[i].Type != "kmtronic" {
+		switch devs[i].Type {
+		case "kmtronic", "denkovi", "webswitch":
+			// known type — keep
+		default:
 			devs[i].Type = "webswitch"
 		}
 		if strings.TrimSpace(devs[i].ID) == "" {