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.

internal/cat/flex.go

@@ -1581,6 +1581,26 @@ func (f *Flex) SetCWBreakInDelay(ms int) error {
 	return nil
 }
 
+// SendCW queues text on the radio's CWX keyer. SmartSDR buffers and keys it, so
+// no WinKeyer / SmartCAT is needed — and because the radio owns the buffer,
+// characters fed while it's already sending append and key in order (the basis
+// for type-ahead). Quotes/backslashes are escaped for the "cwx send \"…\"" form.
+func (f *Flex) SendCW(text string) error {
+	text = strings.TrimRight(text, "\r\n")
+	if strings.TrimSpace(text) == "" {
+		return nil
+	}
+	esc := strings.NewReplacer(`\`, `\\`, `"`, `\"`).Replace(text)
+	f.send(`cwx send "` + esc + `"`)
+	return nil
+}
+
+// StopCW clears the CWX buffer, aborting whatever is currently being keyed.
+func (f *Flex) StopCW() error {
+	f.send("cwx clear")
+	return nil
+}
+
 func (f *Flex) SetCWSidetone(on bool) error {
 	f.mu.Lock()
 	f.tx.cwSidetone = on