feat(auth): required login barrier with 1-click demo access and session management

app/templates/base.html

@@ -15,8 +15,16 @@
         🥨 <span>Schmankerl</span> <small class="tenant-badge">{{ tenant.name if tenant else 'Portal' }}</small>
       </a>
       <nav class="nav-links">
+        {% if current_user %}
         <a href="/">Brotzeit bestellen</a>
+        {% if current_user.role == 'admin' %}
         <a href="/admin" class="nav-admin">👨‍🍳 Einkäufer-Cockpit</a>
+        {% endif %}
+        <span class="user-greeting">👤 {{ current_user.first_name }}</span>
+        <a href="/logout" class="nav-logout">Abmelden</a>
+        {% else %}
+        <a href="/login" class="nav-login">Anmelden</a>
+        {% endif %}
         <a href="https://new.dev.waiswuaschd.de" target="_blank" class="nav-home">Zur Plattform</a>
       </nav>
     </div>