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/cat.go

@@ -394,6 +394,11 @@ type FlexController interface {
 	SetCWSpeed(int) error
 	SetCWPitch(int) error
 	SetCWBreakInDelay(int) error
+	// CWX keyer — buffered CW keying via SmartSDR's CWX subsystem, so a Flex needs
+	// no WinKeyer / SmartCAT. SendCW queues text (the radio buffers and keys it);
+	// StopCW clears the buffer, aborting the send.
+	SendCW(string) error
+	StopCW() error
 	SetCWSidetone(bool) error
 	SetSidetoneLevel(int) error
 	SetCWFilter(int) error