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).

frontend/wailsjs/go/main/App.js

@@ -1654,6 +1654,10 @@ export function TestRotator(arg1) {
   return window['go']['main']['App']['TestRotator'](arg1);
 }
 
+export function TestStationDevice(arg1) {
+  return window['go']['main']['App']['TestStationDevice'](arg1);
+}
+
 export function TestUltrabeam(arg1) {
   return window['go']['main']['App']['TestUltrabeam'](arg1);
 }