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/models.ts

@@ -2746,6 +2746,22 @@ export namespace main {
 	        this.my_pota_ref = source["my_pota_ref"];
 	    }
 	}
+	export class StationTestResult {
+	    ok: boolean;
+	    relays: number;
+	    error?: string;
+	
+	    static createFrom(source: any = {}) {
+	        return new StationTestResult(source);
+	    }
+	
+	    constructor(source: any = {}) {
+	        if ('string' === typeof source) source = JSON.parse(source);
+	        this.ok = source["ok"];
+	        this.relays = source["relays"];
+	        this.error = source["error"];
+	    }
+	}
 	export class ULSStatusResult {
 	    count: number;
 	    updated_at: string;