feat: hide logout button when admin is not logged in

- Add conditional rendering for "Abmelden" button based on isAdminLoggedIn state
- Button only appears when localStorage contains "isAdminLoggedIn": "true"
- Improves UI cleanliness by hiding unnecessary logout option for regular users
- Maintains existing admin warning banner and logout functionality when needed

components/header/Header.tsx

@@ -110,16 +110,17 @@ function Header() {
           */}
         </div>
 
-        {/* Logout-Button */}
-
-        <div className="flex items-center justify-end w-1/4 space-x-1">
-          <button
-            onClick={handleLogout}
-            className="bg-littwin-blue text-white px-4 py-2 rounded"
-          >
-            Abmelden
-          </button>
-        </div>
+        {/* Logout-Button - nur anzeigen wenn Admin eingeloggt ist */}
+        {isAdminLoggedIn && (
+          <div className="flex items-center justify-end w-1/4 space-x-1">
+            <button
+              onClick={handleLogout}
+              className="bg-littwin-blue text-white px-4 py-2 rounded"
+            >
+              Abmelden
+            </button>
+          </div>
+        )}
       </div>
 
       {/* Warnhinweis, wenn der Admin angemeldet ist */}