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.

internal/relaydev/denkovi_other.go

@@ -10,12 +10,17 @@ import (
 	"fmt"
 )
 
-type denkoviStub struct{}
+type denkoviStub struct{ count int }
 
 // NewDenkovi returns a stub on non-Windows builds.
-func NewDenkovi(serial string) Device { return denkoviStub{} }
+func NewDenkovi(serial string, count int) Device {
+	if count != 4 {
+		count = 8
+	}
+	return denkoviStub{count: count}
+}
 
-func (denkoviStub) Count() int { return 8 }
+func (s denkoviStub) Count() int { return s.count }
 func (denkoviStub) Status(context.Context) ([]bool, error) {
 	return nil, fmt.Errorf("Denkovi USB relay board is only supported on Windows")
 }