Add auth.mischlabs.de as wide banner card above service grid

Horizontal banner layout with shield icon, spanning full grid width.
Responsive: collapses to vertical on mobile.

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

index.html

@@ -22,6 +22,20 @@
     </header>
 
     <main>
+        <a href="https://auth.mischlabs.de" class="card card-banner" target="_blank" rel="noopener">
+            <div class="card-icon" style="--accent: #f59e0b;">
+                <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round">
+                    <path d="M12 22s8-4 8-10V5l-8-3-8 3v7c0 6 8 10 8 10z"/>
+                    <path d="M9 12l2 2 4-4"/>
+                </svg>
+            </div>
+            <div class="card-content">
+                <h2>Single Sign-On</h2>
+                <p>Zentrales Login fuer alle Dienste</p>
+            </div>
+            <span class="card-domain">auth.mischlabs.de</span>
+        </a>
+
         <div class="grid">
 
             <a href="https://drive.mischlabs.de" class="card" target="_blank" rel="noopener">