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.d.ts

@@ -194,6 +194,8 @@ export function FlexApplyBandAntenna(arg1:string):Promise<void>;
 
 export function FlexMox(arg1:boolean):Promise<void>;
 
+export function FlexSendCW(arg1:string):Promise<void>;
+
 export function FlexSetAGCMode(arg1:string):Promise<void>;
 
 export function FlexSetAGCThreshold(arg1:number):Promise<void>;
@@ -224,6 +226,8 @@ export function FlexSetCWSpeed(arg1:number):Promise<void>;
 
 export function FlexSetFilter(arg1:number,arg2:number):Promise<void>;
 
+export function FlexSetKeySpeed(arg1:number):Promise<void>;
+
 export function FlexSetMic(arg1:number):Promise<void>;
 
 export function FlexSetMicProfile(arg1:string):Promise<void>;
@@ -280,6 +284,8 @@ export function FlexSetXIT(arg1:boolean):Promise<void>;
 
 export function FlexSetXITFreq(arg1:number):Promise<void>;
 
+export function FlexStopCW():Promise<void>;
+
 export function FlexTune(arg1:boolean):Promise<void>;
 
 export function GetADIFMonitor():Promise<main.ADIFMonitorConfig>;