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/wailsjs/go/models.ts

@@ -2298,6 +2298,7 @@ export namespace main {
 	    backend: string;
 	    omnirig_rig: number;
 	    omnirig_vfo: string;
+	    digi_as_usb: boolean;
 	    flex_host: string;
 	    flex_port: number;
 	    flex_spots: boolean;
@@ -2350,6 +2351,7 @@ export namespace main {
 	        this.backend = source["backend"];
 	        this.omnirig_rig = source["omnirig_rig"];
 	        this.omnirig_vfo = source["omnirig_vfo"];
+	        this.digi_as_usb = source["digi_as_usb"];
 	        this.flex_host = source["flex_host"];
 	        this.flex_port = source["flex_port"];
 	        this.flex_spots = source["flex_spots"];