Fix broken pages and SEO

[tmshv]

Problems

1. 404 page renders "undefined" 24 times — src/app/not-found.tsx:5
   Component expects statusCode prop but App Router not-found.tsx receives no props. Replace with:

   export default function NotFound() {
     // ...
     <Line line={Array(24).fill('404')} />
   }

2. Sitemap has placeholder "acme.com" URLs — src/app/sitemap.ts
   Contains Next.js template defaults pointing to acme.com with routes that don't exist (/about, /blog). Generate dynamically from getPages() with base URL https://unit4.io.

3. Hardcoded copyright year — src/app/layout.tsx:86
   Footer says 2017-2025. Use new Date().getFullYear() for the end year.