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

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

@@ -9,6 +9,7 @@ import { Badge } from '@kit/ui/badge';
 import { Button } from '@kit/ui/button';
 import { Card, CardContent, CardHeader, CardTitle } from '@kit/ui/card';
 
+import { AccountNotFound } from '~/components/account-not-found';
 import { CmsPageShell } from '~/components/cms-page-shell';
 import { EmptyState } from '~/components/empty-state';
 
@@ -43,7 +44,7 @@ export default async function ParticipantsPage({ params }: PageProps) {
     api.getParticipants(courseId),
   ]);
 
-  if (!course) return <div>Kurs nicht gefunden</div>;
+  if (!course) return <AccountNotFound />;
 
   return (
     <CmsPageShell account={account} title="Teilnehmer">
@@ -56,7 +57,7 @@ export default async function ParticipantsPage({ params }: PageProps) {
               {participants.length} Teilnehmer
             </p>
           </div>
-          <Button>
+          <Button data-test="participants-add-btn">
             <Plus className="mr-2 h-4 w-4" />
             Teilnehmer anmelden
           </Button>