feat: a level control for the voice keyer, and name every CAT backend in its PTT list

The messages went to the radio exactly as recorded. Nothing in OpsLog could
raise them, so a microphone captured quietly drove the rig quietly and the only
remedies were the radio's own USB input menu or the Windows mixer — which is
where the operator was heading. There is now a level from 10 to 400 %, applied
with clamping (a wrap would turn loud speech into noise on the air), and Play
previews at that same level so the adjustment is made against what will actually
be transmitted.

The PTT method list also named only OmniRig, Flex, Icom and TCI, so choosing a
native Yaesu left "CAT" with no backend beside it — which reads as "there is no
CAT PTT for my radio" and sends the operator to RTS on a COM port that has
nothing to do with the rig. Same list-needing-every-member shape as three
earlier bugs in this feature. The TestPTT log line had the same rot: it said
"CAT via OmniRig" whatever backend was running.

frontend/wailsjs/go/models.ts

@@ -1601,6 +1601,7 @@ export namespace main {
 	    format: string;
 	    from_gain: number;
 	    mic_gain: number;
+	    tx_gain: number;
 	
 	    static createFrom(source: any = {}) {
 	        return new AudioSettings(source);
@@ -1620,6 +1621,7 @@ export namespace main {
 	        this.format = source["format"];
 	        this.from_gain = source["from_gain"];
 	        this.mic_gain = source["mic_gain"];
+	        this.tx_gain = source["tx_gain"];
 	    }
 	}
 	export class AutostartLaunchResult {