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/src/components/StationControlPanel.tsx

@@ -12,7 +12,7 @@ import {
   GetStationDevices, SaveStationDevices, GetStationStatus, StationSetRelay,
   GetRotatorHeading, RotatorGoTo, RotatorStop,
   GetUltrabeamStatus, SetUltrabeamDirection, UltrabeamRetract, MotorSetElement, MotorReadElements,
-  ListDenkoviDevices, ListSerialPorts,
+  ListDenkoviDevices, ListSerialPorts, TestStationDevice,
 } from '../../wailsjs/go/main/App';
 
 type RotatorProps = { centerLat?: number | null; centerLon?: number | null; bearing?: number | null };
@@ -473,16 +473,33 @@ function DeviceEditor({ device, onChange, onSave, onCancel, t }: {
   // Detected FTDI serials for the Denkovi picker.
   const [denkoviSerials, setDenkoviSerials] = useState<string[]>([]);
   const [detecting, setDetecting] = useState(false);
+  const [detectMsg, setDetectMsg] = useState('');
   const detectDenkovi = async () => {
     setDetecting(true);
+    setDetectMsg('');
     try {
       const list = ((await ListDenkoviDevices()) ?? []) as string[];
       setDenkoviSerials(list);
       // Auto-fill when there's exactly one and nothing chosen yet.
       if (list.length >= 1 && !device.host.trim()) onChange({ ...device, host: list[0] });
-    } catch { setDenkoviSerials([]); }
+      setDetectMsg(list.length === 0 ? t('station.detectNone') : t('station.detectFound', { n: list.length }));
+    } catch (e: any) { setDenkoviSerials([]); setDetectMsg(String(e?.message || e || t('station.detectNone'))); }
     finally { setDetecting(false); }
   };
+  // Connection test result for the current device config.
+  const [testing, setTesting] = useState(false);
+  const [testMsg, setTestMsg] = useState<{ ok: boolean; text: string } | null>(null);
+  const testDevice = async () => {
+    setTesting(true);
+    setTestMsg(null);
+    try {
+      const r: any = await TestStationDevice(device as any);
+      setTestMsg(r?.ok
+        ? { ok: true, text: t('station.testOk', { n: r.relays }) }
+        : { ok: false, text: r?.error || t('station.testFail') });
+    } catch (e: any) { setTestMsg({ ok: false, text: String(e?.message || e || t('station.testFail')) }); }
+    finally { setTesting(false); }
+  };
   useEffect(() => { if (isDenkovi) void detectDenkovi(); /* eslint-disable-next-line */ }, [isDenkovi]);
   return (
     <div ref={ref} className="mt-4 max-w-4xl rounded-xl border border-primary/40 bg-card p-4 space-y-3">
@@ -534,6 +551,7 @@ function DeviceEditor({ device, onChange, onSave, onCancel, t }: {
             </Button>
           </div>
           <p className="text-[10px] text-muted-foreground">{t('station.ftdiHint')}</p>
+          {detectMsg && <p className="text-[11px] font-medium text-muted-foreground">{detectMsg}</p>}
         </div>
       ) : isUsbRelay ? (
         <div className="space-y-1 max-w-md">
@@ -581,9 +599,21 @@ function DeviceEditor({ device, onChange, onSave, onCancel, t }: {
           ))}
         </div>
       </div>
-      <div className="flex justify-end gap-2">
-        <Button size="sm" variant="ghost" onClick={onCancel}><X className="size-3.5 mr-1" />{t('station.cancel')}</Button>
-        <Button size="sm" onClick={onSave} disabled={!device.host.trim()}><Check className="size-3.5 mr-1" />{t('station.save')}</Button>
+      <div className="flex items-center gap-2">
+        {testMsg && (
+          <span className={cn('inline-flex items-center gap-1.5 text-xs font-medium', testMsg.ok ? 'text-success' : 'text-danger')}>
+            <span className={cn('size-2 rounded-full', testMsg.ok ? 'bg-success' : 'bg-danger')} />
+            {testMsg.text}
+          </span>
+        )}
+        <div className="ml-auto flex gap-2">
+          <Button size="sm" variant="outline" onClick={testDevice} disabled={testing || !device.host.trim()}>
+            {testing ? <Loader2 className="size-3.5 mr-1 animate-spin" /> : <PlugZap className="size-3.5 mr-1" />}
+            {t('station.test')}
+          </Button>
+          <Button size="sm" variant="ghost" onClick={onCancel}><X className="size-3.5 mr-1" />{t('station.cancel')}</Button>
+          <Button size="sm" onClick={onSave} disabled={!device.host.trim()}><Check className="size-3.5 mr-1" />{t('station.save')}</Button>
+        </div>
       </div>
     </div>
   );