feat: enhance accessibility and testing with data-test attributes and improve error handling

apps/web/app/[locale]/home/[account]/courses/page.tsx

@@ -64,7 +64,7 @@ export default async function CoursesPage({ params, searchParams }: PageProps) {
           <p className="text-muted-foreground">Kursangebot verwalten</p>
 
           <Link href={`/home/${account}/courses/new`}>
-            <Button>
+            <Button data-test="courses-new-btn">
               <Plus className="mr-2 h-4 w-4" />
               Neuer Kurs
             </Button>