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_windows.go

@@ -121,6 +121,19 @@ func (d *denkovi) Set(ctx context.Context, relay int, on bool) error {
 	return d.writeLocked()
 }
 
+// Close releases the FTDI handle so the board can be reopened later (only one
+// handle may hold a D2XX device at a time).
+func (d *denkovi) Close() error {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	if d.opened {
+		procClose.Call(d.h)
+		d.opened = false
+		d.h = 0
+	}
+	return nil
+}
+
 func (d *denkovi) Status(ctx context.Context) ([]bool, error) {
 	d.mu.Lock()
 	defer d.mu.Unlock()