feat: Help - What's new button to reopen the changelog anytime

Adds GetChangelog (all entries up to the running version, no seen-marker change)
and a Help menu 'What's new' item that reopens the changelog dialog on demand.

frontend/wailsjs/go/main/App.d.ts

@@ -325,6 +325,8 @@ export function GetCATState():Promise<cat.RigState>;
 
 export function GetCatalogCodes():Promise<Array<string>>;
 
+export function GetChangelog():Promise<Array<main.ChangelogEntry>>;
+
 export function GetChatHistory(arg1:number):Promise<Array<main.ChatMessage>>;
 
 export function GetClublogCtyInfo():Promise<main.ClublogCtyInfo>;