feat: What's-new changelog dialog on first launch after an update (EN/FR)

Embeds changelog.json (per-version EN + FR notes, curated from the release's
commits). GetWhatsNew compares the stored last-seen version with the running
build and returns the notes for every newer version, once, then advances the
marker. A dialog shows them in the UI language on the first launch after update.
Seeded with the 0.20.5 notes.

frontend/wailsjs/go/main/App.js

@@ -810,6 +810,10 @@ export function GetUltrabeamStatus() {
   return window['go']['main']['App']['GetUltrabeamStatus']();
 }
 
+export function GetWhatsNew() {
+  return window['go']['main']['App']['GetWhatsNew']();
+}
+
 export function GetWinkeyerSettings() {
   return window['go']['main']['App']['GetWinkeyerSettings']();
 }