feat: navigate to mailbox after successful mail import

- Add onSuccess prop to MailImportWizard, called instead of onClose on success
- MailDetailPage navigates to /mailpostfach after import completes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-frontend/src/pages/MailDetailPage.tsx

@@ -252,11 +252,12 @@ export default function MailDetailPage() {
       </div>
 
       {wizardOpen && email && (
-        <MailImportWizard 
-          visible={wizardOpen} 
-          onClose={() => setWizardOpen(false)} 
-          email={email} 
-          attachments={attachments} 
+        <MailImportWizard
+          visible={wizardOpen}
+          onClose={() => setWizardOpen(false)}
+          onSuccess={() => navigate('/mailpostfach')}
+          email={email}
+          attachments={attachments}
         />
       )}
     </div>