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.

frontend/wailsjs/go/main/App.js

@@ -346,6 +346,10 @@ export function FlexMox(arg1) {
   return window['go']['main']['App']['FlexMox'](arg1);
 }
 
+export function FlexSendCW(arg1) {
+  return window['go']['main']['App']['FlexSendCW'](arg1);
+}
+
 export function FlexSetAGCMode(arg1) {
   return window['go']['main']['App']['FlexSetAGCMode'](arg1);
 }
@@ -406,6 +410,10 @@ export function FlexSetFilter(arg1, arg2) {
   return window['go']['main']['App']['FlexSetFilter'](arg1, arg2);
 }
 
+export function FlexSetKeySpeed(arg1) {
+  return window['go']['main']['App']['FlexSetKeySpeed'](arg1);
+}
+
 export function FlexSetMic(arg1) {
   return window['go']['main']['App']['FlexSetMic'](arg1);
 }
@@ -518,6 +526,10 @@ export function FlexSetXITFreq(arg1) {
   return window['go']['main']['App']['FlexSetXITFreq'](arg1);
 }
 
+export function FlexStopCW() {
+  return window['go']['main']['App']['FlexStopCW']();
+}
+
 export function FlexTune(arg1) {
   return window['go']['main']['App']['FlexTune'](arg1);
 }