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.d.ts

@@ -427,6 +427,8 @@ export function GetUltrabeamSettings():Promise<main.UltrabeamSettings>;
 
 export function GetUltrabeamStatus():Promise<main.UltrabeamStatusInfo>;
 
+export function GetWhatsNew():Promise<Array<main.ChangelogEntry>>;
+
 export function GetWinkeyerSettings():Promise<main.WinkeyerSettings>;
 
 export function GetWinkeyerStatus():Promise<winkeyer.Status>;