feat: Redirect to /menu if admin is already logged in on login page

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

frontend/login.html

@@ -46,6 +46,13 @@
   </div>
   <script src="/static/header.js"></script>
   <script>
+    (async function() {
+      try {
+        const r = await fetch('/api/albums');
+        if (r.ok) location.href = '/menu';
+      } catch (e) {}
+    })();
+
     const form = document.getElementById('loginForm');
     const msg = document.getElementById('msg');
     function show(kind, text){