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/src/components/SettingsModal.tsx

@@ -2807,6 +2807,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                 <SelectContent>
                   <SelectItem value="winkeyer">WinKeyer (serial)</SelectItem>
                   <SelectItem value="icom">Icom CI-V (rig keyer)</SelectItem>
+                  <SelectItem value="flex">FlexRadio (CWX)</SelectItem>
                   <SelectItem value="tci" disabled>TCI (coming soon)</SelectItem>
                 </SelectContent>
               </Select>
@@ -2837,6 +2838,26 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                 </div>
               </div>
             </>
+          ) : wk.engine === 'flex' ? (
+            <>
+              <p className="text-xs text-muted-foreground -mt-2">
+                FlexRadio keys CW through the radio's <strong>CWX</strong> keyer over the existing SmartSDR CAT connection — no WinKeyer or SmartCAT needed. It reuses the connection set in Settings → CAT, so there's nothing else to wire up here. Put a slice in CW mode. Only the speed is set from here; weight, sidetone and break-in are configured on the radio (break-in must be on for CW to actually transmit).
+              </p>
+              {(!catCfg.enabled || catCfg.backend !== 'flex') && (
+                <p className="text-xs font-medium text-warning -mt-1 flex items-start gap-1.5">
+                  <span aria-hidden>⚠</span>
+                  <span>
+                    Your CAT backend is set to <strong>{catCfg.enabled ? (catCfg.backend || 'none') : 'disabled'}</strong>. Flex CWX needs the CAT backend set to <strong>FlexRadio</strong> and connected — change it under Settings → CAT interface, otherwise sending CW will fail.
+                  </span>
+                </p>
+              )}
+              <div className="grid grid-cols-4 gap-3">
+                <div className="space-y-1">
+                  <Label>Speed (WPM)</Label>
+                  <Input type="number" min={6} max={48} value={wk.wpm} onChange={(e) => setWkField({ wpm: num(e.target.value, 25) })} className="font-mono" />
+                </div>
+              </div>
+            </>
           ) : (
             <>
               <div className="grid grid-cols-4 gap-3">