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/src/App.tsx

@@ -36,7 +36,7 @@ import {
   GetWinkeyerSettings, SaveWinkeyerSettings, ListSerialPorts, GetWinkeyerStatus,
   WinkeyerConnect, WinkeyerDisconnect, WinkeyerSend, WinkeyerStop, WinkeyerSetSpeed, WinkeyerBackspace,
   IcomSendCW, IcomStopCW, IcomSetKeySpeed, IcomSetBreakIn, GetIcomState,
-  FlexSendCW, FlexStopCW, FlexSetKeySpeed,
+  FlexSendCW, FlexStopCW, FlexSetKeySpeed, FlexBackspaceCW,
   GetDVKMessages, GetDVKStatus, DVKPlay, DVKStop,
   StartCWDecoder, StopCWDecoder, SetCWDecoderPitch,
   ChatAvailable, GetChatHistory, SendChatMessage, GetOnlineOperators,
@@ -2221,7 +2221,10 @@ export default function App() {
     if (cwSourceRef.current === 'flex') { FlexSendCW(chars).catch(() => {}); return; }
     WinkeyerSend(chars).catch(() => {});
   }
-  function wkBackspace() { WinkeyerBackspace().catch(() => {}); }
+  function wkBackspace() {
+    if (cwSourceRef.current === 'flex') { FlexBackspaceCW(1).catch(() => {}); return; }
+    WinkeyerBackspace().catch(() => {});
+  }
   function wkToggleSendOnType(on: boolean) { setWkSendOnType(on); saveWk({ send_on_type: on }); }
 
   // Resolve slot status for any spot we haven't seen yet — debounced so we