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.d.ts

@@ -849,6 +849,8 @@ export function TestQRZUpload():Promise<string>;
 
 export function TestRotator(arg1:main.RotatorSettings):Promise<void>;
 
+export function TestStationDevice(arg1:main.StationDevice):Promise<main.StationTestResult>;
+
 export function TestUltrabeam(arg1:main.UltrabeamSettings):Promise<void>;
 
 export function ULSStatus():Promise<main.ULSStatusResult>;