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

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

@@ -41,7 +41,7 @@ export default async function NewsletterDetailPage({ params }: PageProps) {
     api.getRecipients(campaignId),
   ]);
 
-  if (!newsletter) return <div>Newsletter nicht gefunden</div>;
+  if (!newsletter) return <AccountNotFound />;
 
   const status = String(newsletter.status);
   const sentCount = recipients.filter(
@@ -60,6 +60,7 @@ export default async function NewsletterDetailPage({ params }: PageProps) {
           <Link
             href={`/home/${account}/newsletter`}
             className="text-muted-foreground hover:text-foreground inline-flex items-center text-sm"
+            data-test="newsletter-back-link"
           >
             <ArrowLeft className="mr-1 h-4 w-4" />
             Zurück zu Newsletter
@@ -98,7 +99,7 @@ export default async function NewsletterDetailPage({ params }: PageProps) {
             {/* Actions */}
             {status === 'draft' && (
               <div className="mt-6">
-                <Button>
+                <Button data-test="newsletter-send-btn">
                   <Send className="mr-2 h-4 w-4" />
                   Newsletter versenden
                 </Button>