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.

app.go

@@ -178,7 +178,8 @@ const (
 	keyRotatorType         = "rotator.type"      // "pst" (PstRotator UDP) | "rotgenius" (4O3A native TCP) | "arco" (microHAM ARCO, GS-232A)
 	keyRotatorNum          = "rotator.num"       // Rotator Genius rotator index: 1 or 2
 	keyRotatorTransport    = "rotator.transport" // arco: "tcp" (LAN) | "serial" (USB virtual COM)
-	keyRotatorComPort      = "rotator.com_port"  // arco serial transport
+	keyRotatorComPort      = "rotator.com_port"  // GS-232 serial transport
+	keyRotatorBaud         = "rotator.baud"      // GS-232 serial baud (an ERC needs it; an ARCO ignores it)
 
 	// Motorized antenna (Ultrabeam or SteppIR) — Hardware → Antenna. Keys keep the
 	// "ultrabeam." prefix for backward compatibility with configs saved before the
@@ -12550,8 +12551,11 @@ type RotatorSettings struct {
 	Port         int    `json:"port"`          // default 12000 (pst) / 9006 (rotgenius) / 4001 (arco — must match the port set in ARCO's LAN CONTROL PROTOCOL)
 	HasElevation bool   `json:"has_elevation"` // include EL in GoTo packets (PstRotator)
 	RotatorNum   int    `json:"rotator_num"`   // Rotator Genius index: 1 or 2
-	Transport    string `json:"transport"`     // arco: "tcp" (LAN) | "serial" (USB virtual COM)
-	ComPort      string `json:"com_port"`      // arco serial transport
+	Transport    string `json:"transport"`     // GS-232 controller: "tcp" (LAN) | "serial" (COM)
+	ComPort      string `json:"com_port"`      // GS-232 serial transport
+	// Baud for the serial transport. An ARCO's virtual COM ignores it; an ERC runs
+	// at the rate set in its own configuration, so it has to be selectable.
+	Baud int `json:"baud"`
 }
 
 // GetRotatorSettings returns the persisted rotator config with defaults.
@@ -12562,7 +12566,7 @@ func (a *App) GetRotatorSettings() (RotatorSettings, error) {
 	}
 	m, err := a.settings.GetMany(a.ctx,
 		keyRotatorEnabled, keyRotatorHost, keyRotatorPort, keyRotatorHasElevation, keyRotatorType, keyRotatorNum,
-		keyRotatorTransport, keyRotatorComPort)
+		keyRotatorTransport, keyRotatorComPort, keyRotatorBaud)
 	if err != nil {
 		return out, err
 	}
@@ -12589,6 +12593,10 @@ func (a *App) GetRotatorSettings() (RotatorSettings, error) {
 		out.Transport = "serial"
 	}
 	out.ComPort = m[keyRotatorComPort]
+	out.Baud = 9600
+	if b, _ := strconv.Atoi(m[keyRotatorBaud]); b > 0 {
+		out.Baud = b
+	}
 	return out, nil
 }
 
@@ -12622,6 +12630,7 @@ func (a *App) SaveRotatorSettings(s RotatorSettings) error {
 		keyRotatorNum:          strconv.Itoa(s.RotatorNum),
 		keyRotatorTransport:    s.Transport,
 		keyRotatorComPort:      strings.TrimSpace(s.ComPort),
+		keyRotatorBaud:         strconv.Itoa(s.Baud),
 	} {
 		if err := a.settings.Set(a.ctx, k, v); err != nil {
 			return err
@@ -12634,7 +12643,7 @@ func (a *App) SaveRotatorSettings(s RotatorSettings) error {
 // USB virtual COM (serial) or the LAN CONTROL PROTOCOL TCP port.
 func arcoClient(s RotatorSettings) *gs232.Client {
 	if s.Transport == "serial" {
-		return gs232.NewSerial(s.ComPort)
+		return gs232.NewSerial(s.ComPort, s.Baud)
 	}
 	return gs232.New(s.Host, s.Port)
 }