feat: FlexRadio CWX CW keyer (send/stop/speed) — no WinKeyer/SmartCAT needed

Adds a third CW engine alongside WinKeyer and Icom CI-V: FlexRadio's CWX keyer
over the existing SmartSDR CAT connection.

- cat.FlexController: SendCW (cwx send), StopCW (cwx clear); flex.go implements
  them, escaping the quoted text form. Speed reuses SetCWSpeed (cw wpm).
- App bindings FlexSendCW/FlexStopCW/FlexSetKeySpeed via FlexDo.
- Frontend: cwSource gains 'flex'; macros/auto-call/<LOGQSO>/send-on-type route to
  Flex when the CAT backend is a Flex (estimate-based timing like Icom, no busy
  echo). Keyer panel shows 'Flex CWX' + ready/offline; Settings adds the engine
  option with a CAT-backend guard.

Core send/stop/speed first; type-ahead buffer + mid-send backspace (cwx delete)
to follow.

app.go

@@ -10359,6 +10359,36 @@ func (a *App) IcomSendCW(text string) error {
 	return err
 }
 
+// FlexSendCW keys a CW message through the FlexRadio CWX keyer (SmartSDR), so a
+// Flex needs no WinKeyer / SmartCAT. Text is already variable-resolved by the UI.
+func (a *App) FlexSendCW(text string) error {
+	if a.cat == nil {
+		return fmt.Errorf("cat not initialized")
+	}
+	err := a.cat.FlexDo(func(fc cat.FlexController) error { return fc.SendCW(text) })
+	if err != nil {
+		applog.Printf("flex cw: FlexSendCW(%q) failed: %v", text, err)
+	}
+	return err
+}
+
+// FlexStopCW clears the CWX buffer, aborting whatever is being keyed.
+func (a *App) FlexStopCW() error {
+	if a.cat == nil {
+		return fmt.Errorf("cat not initialized")
+	}
+	return a.cat.FlexDo(func(fc cat.FlexController) error { return fc.StopCW() })
+}
+
+// FlexSetKeySpeed sets the CW keyer speed in WPM (the CWX keyer uses the radio's
+// CW speed).
+func (a *App) FlexSetKeySpeed(wpm int) error {
+	if a.cat == nil {
+		return fmt.Errorf("cat not initialized")
+	}
+	return a.cat.FlexDo(func(fc cat.FlexController) error { return fc.SetCWSpeed(wpm) })
+}
+
 // IcomStopCW aborts the CW message currently being sent.
 func (a *App) IcomStopCW() error {
 	if a.cat == nil {