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.

frontend/wailsjs/go/main/App.js

@@ -342,6 +342,10 @@ export function FlexApplyBandAntenna(arg1) {
   return window['go']['main']['App']['FlexApplyBandAntenna'](arg1);
 }
 
+export function FlexBackspaceCW(arg1) {
+  return window['go']['main']['App']['FlexBackspaceCW'](arg1);
+}
+
 export function FlexMox(arg1) {
   return window['go']['main']['App']['FlexMox'](arg1);
 }