feat: add application toast messages

index.html

@@ -9,6 +9,14 @@
   </head>
   <body>
     <div class="app-shell">
+      <div
+        id="message-region"
+        class="message-region"
+        aria-label="Application messages"
+        aria-live="polite"
+        aria-relevant="additions"
+      ></div>
+
       <button
         id="sidebar-open"
         class="sidebar-toggle sidebar-open"