feat: Flex CWX type-ahead backspace (cwx erase) — phase 2

Completes the CWX type-ahead loop: with send-on-type the keyer-panel CW text
already streams each typed char to the radio's CWX buffer (which keys in order,
so you can keep typing while it sends); this adds the matching un-send. Route
wkBackspace to FlexBackspaceCW -> cwx erase N, so backspacing a mistyped char in
send-on-type removes it from the buffer before the radio keys it.

app.go

@@ -10389,6 +10389,15 @@ func (a *App) FlexSetKeySpeed(wpm int) error {
 	return a.cat.FlexDo(func(fc cat.FlexController) error { return fc.SetCWSpeed(wpm) })
 }
 
+// FlexBackspaceCW removes the last n not-yet-keyed characters from the CWX buffer
+// (type-ahead correction). n<1 deletes one.
+func (a *App) FlexBackspaceCW(n int) error {
+	if a.cat == nil {
+		return fmt.Errorf("cat not initialized")
+	}
+	return a.cat.FlexDo(func(fc cat.FlexController) error { return fc.BackspaceCW(n) })
+}
+
 // IcomStopCW aborts the CW message currently being sent.
 func (a *App) IcomStopCW() error {
 	if a.cat == nil {