Add loading indicator

assets/page.erb

@@ -129,6 +129,20 @@
     .logout-btn:hover {
       background: #2a3040;
     }
+
+    .spinner {
+      width: 24px;
+      height: 24px;
+      margin: 1rem auto;
+      border: 3px solid #1e2433;
+      border-top-color: #4ade80;
+      border-radius: 50%;
+      animation: spin 0.8s linear infinite;
+    }
+
+    @keyframes spin {
+      to { transform: rotate(360deg); }
+    }
   </style>
 </head>
 <body>
@@ -145,7 +159,7 @@
 </header>
 
 <% if authenticated %>
-  <div id="content"></div>
+  <div id="content"><div class="spinner"></div></div>
   <script>
     fetch("?content", { credentials: "same-origin" })
       .then(function(response) {