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>

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

@@ -82,7 +82,7 @@ export default async function NewsletterPage({
           </div>
 
           <Link href={`/home/${account}/newsletter/new`}>
-            <Button>
+            <Button data-test="newsletter-new-btn">
               <Plus className="mr-2 h-4 w-4" />
               Neuer Newsletter
             </Button>

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

@@ -43,7 +43,7 @@ export default async function NewsletterTemplatesPage({ params }: PageProps) {
             </p>
           </div>
 
-          <Button>
+          <Button data-test="newsletter-templates-new-btn">
             <Plus className="mr-2 h-4 w-4" />
             Neue Vorlage
           </Button>