overhaul chat structure

public/app/workspaceNavigation.js

@@ -0,0 +1,17 @@
+import {post} from './api.js';
+
+// Every page gets this control from the same server-rendered header.
+for (const button of document.querySelectorAll('[data-sign-out]')) {
+    button.addEventListener('click', async () => {
+        button.disabled = true;
+        const result = await post('/logout', {});
+        if (result.error) {
+            button.disabled = false;
+            const notice = document.querySelector('#shellNotice');
+            notice.textContent = result.error;
+            notice.classList.remove('hidden');
+            return;
+        }
+        location.assign('/');
+    });
+}