feat: support ERC rotator controllers through the existing GS-232A backend

An ERC (Easy Rotor Control, incl. ERC Mini) emulates Yaesu GS-232A/B, which is
exactly what the backend written for the microHAM ARCO already speaks — azimuth
out (Maaa), azimuth in (C), stop (S), which is the whole of what was asked for.
So this is two small gaps rather than a new backend:

  - Serial speed was hardcoded to 9600. An ARCO's virtual COM ignores it, but an
    ERC runs at whatever its own configuration sets, and a mismatch reads as a
    dead rotator. It is now selectable beside the COM port.
  - The entry was called "microHAM ARCO", so an ERC owner would never have found
    it. It is named after the PROTOCOL now: "GS-232A controller (microHAM ARCO,
    ERC…)".

The help text states the condition plainly in both languages, because it is the
one thing that will otherwise waste an evening: an ERC left on Hy-Gain DCU-1
speaks a different command set and simply will not answer.

Untested on hardware — I have no ERC here, and the GS-232 path itself is proven
on a real ARCO.

frontend/wailsjs/go/models.ts

@@ -2799,6 +2799,7 @@ export namespace main {
 	    rotator_num: number;
 	    transport: string;
 	    com_port: string;
+	    baud: number;
 	
 	    static createFrom(source: any = {}) {
 	        return new RotatorSettings(source);
@@ -2814,6 +2815,7 @@ export namespace main {
 	        this.rotator_num = source["rotator_num"];
 	        this.transport = source["transport"];
 	        this.com_port = source["com_port"];
+	        this.baud = source["baud"];
 	    }
 	}
 	export class ScpStatus {