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/serialrelay.go

@@ -37,6 +37,19 @@ func NewSerialRelay(port string, count int) Device {
 
 func (s *serialRelay) Count() int { return s.count }
 
+// Close releases the COM port so it can be reopened later (only one handle may
+// hold a serial port at a time).
+func (s *serialRelay) Close() error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.port != nil {
+		err := s.port.Close()
+		s.port = nil
+		return err
+	}
+	return nil
+}
+
 func (s *serialRelay) ensureOpen() error {
 	if s.port != nil {
 		return nil