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/models.ts

@@ -1728,6 +1728,24 @@ export namespace main {
 	        this.path = source["path"];
 	    }
 	}
+	export class ChangelogEntry {
+	    version: string;
+	    date: string;
+	    en: string[];
+	    fr: string[];
+	
+	    static createFrom(source: any = {}) {
+	        return new ChangelogEntry(source);
+	    }
+	
+	    constructor(source: any = {}) {
+	        if ('string' === typeof source) source = JSON.parse(source);
+	        this.version = source["version"];
+	        this.date = source["date"];
+	        this.en = source["en"];
+	        this.fr = source["fr"];
+	    }
+	}
 	export class ChatMessage {
 	    id: number;
 	    operator: string;