fix: cache relay-board drivers so Denkovi/USB-serial boards stay connected

The station-control code rebuilt a fresh driver on every status poll and every
relay set. Stateful boards (Denkovi FTDI D2XX, USB-serial) hold an OS handle only
one opener can own, so the first poll opened the board and leaked the handle, and
every poll after failed with 'device in use' — the relays greyed out a second
after Save and auto-control never switched. Drivers are now cached per device and
reused, closed on config change. Adds a Test-connection button + detect feedback
in the device editor (reported by VK4MA).

internal/relaydev/denkovi_other.go

@@ -20,7 +20,8 @@ func NewDenkovi(serial string, count int) Device {
 	return denkoviStub{count: count}
 }
 
-func (s denkoviStub) Count() int { return s.count }
+func (s denkoviStub) Count() int     { return s.count }
+func (denkoviStub) Close() error     { return nil }
 func (denkoviStub) Status(context.Context) ([]bool, error) {
 	return nil, fmt.Errorf("Denkovi USB relay board is only supported on Windows")
 }