feat(cat): USB for digital modes, on every backend

A soundcard mode is USB with audio in the mic path, and that is what
most rigs need. Asking for the mode by NAME is the better answer on a
modern transceiver with a DATA position and the wrong one on an older
set, where the CAT layer resolves 'digital' to RTTY/FSK — OmniRig does
exactly that, per rig file, and there is no arguing with it from here.
The operator clicking an FT8 spot landed in FSK, which keys from a
mark/space generator and cannot pass FT8 at all.

Applied in SetCATMode, the one funnel every backend and every caller
goes through, so it holds for a spot click, the band map and the mode
selector alike. The QSO is still logged as FT8: this is what the radio
is put in, not what the contact was.

frontend/src/components/SettingsModal.tsx

@@ -1612,7 +1612,7 @@ function SettingsModalImpl({ onClose, onSaved, initialSection, onMainPaneChanged
     icom_port: '', icom_baud: 115200, icom_addr: 0x98, icom_net_host: '', icom_net_user: '', icom_net_pass: '', icom_net_audio: false,
     tci_host: '', tci_port: 40001, tci_spots: false, poll_ms: 250, delay_ms: 0, offset_on: false, offset_hz: 0,
     digital_default: 'FT8', share_enabled: false, share_port: 4532, share_proto: 'rigctl', share_tci_port: 40001,
-    ptt_hotkey_enabled: false, ptt_hotkey: '', ptt_hotkey_toggle: false,
+    ptt_hotkey_enabled: false, ptt_hotkey: '', ptt_hotkey_toggle: false, digi_as_usb: false,
   });
   // Brand + connection, derived from the stored backend rather than held
   // separately: two sources for one fact drift apart the first time something
@@ -3840,6 +3840,13 @@ function SettingsModalImpl({ onClose, onSaved, initialSection, onMainPaneChanged
                 </label>              </>
             )}
           </div>
+          <label className="flex items-center gap-2 text-sm cursor-pointer" title={t('cat.digiUsbHint')}>
+            <Checkbox
+              checked={!!catCfg.digi_as_usb}
+              onCheckedChange={(c) => setCatCfg((s) => ({ ...s, digi_as_usb: !!c }))}
+            />
+            {t('cat.digiUsb')}
+          </label>
           {catCfg.backend === 'omnirig' && (
           <>
           <label className="flex items-center gap-2 text-sm cursor-pointer">