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/src/components/SettingsModal.tsx

@@ -3198,7 +3198,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                 <SelectContent>
                   <SelectItem value="pst">PstRotator (UDP)</SelectItem>
                   <SelectItem value="rotgenius">Rotator Genius (4O3A, native)</SelectItem>
-                  <SelectItem value="arco">microHAM ARCO (LAN, GS-232A)</SelectItem>
+                  <SelectItem value="arco">GS-232A controller (microHAM ARCO, ERC…)</SelectItem>
                 </SelectContent>
               </Select>
             </div>
@@ -3245,6 +3245,15 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                 <Button size="sm" variant="outline" className="h-9" onClick={() => ListSerialPorts().then((p) => setWkPorts((p ?? []) as string[])).catch(() => {})}>
                   <ArrowDown className="size-3.5 rotate-90" />
                 </Button>
+                <Select value={String((rotator as any).baud || 9600)}
+                  onValueChange={(v) => setRotator((s) => ({ ...s, baud: Number(v) } as any))}>
+                  <SelectTrigger className="h-9 w-28"><SelectValue /></SelectTrigger>
+                  <SelectContent>
+                    {[4800, 9600, 19200, 38400, 57600].map((b) => (
+                      <SelectItem key={b} value={String(b)}>{b} baud</SelectItem>
+                    ))}
+                  </SelectContent>
+                </Select>
               </div>
             </div>
           ) : (