fix(qsl): cancel an in-flight confirmation download on close / new download

The LoTW/QRZ confirmation download ran on the app-lifetime context, so it
kept going after the QSL Manager was closed and a new download didn't stop the
previous one. A slow QRZ sync therefore bled its "flag cleared" log into a
freshly started LoTW download — the operator saw QRZ activity during a LoTW
run. Now each download gets a cancellable context: starting one cancels the
previous, closing the window cancels it (CancelConfirmations), and the parse
loops bail promptly on cancel. (A LoTW http 503 is separate — ARRL's server.)

frontend/wailsjs/go/main/App.js

@@ -142,6 +142,10 @@ export function CWDecoderRunning() {
   return window['go']['main']['App']['CWDecoderRunning']();
 }
 
+export function CancelConfirmations() {
+  return window['go']['main']['App']['CancelConfirmations']();
+}
+
 export function ChatAvailable() {
   return window['go']['main']['App']['ChatAvailable']();
 }