UI: collapsible sections in help modal

Post-processes rendered markdown: each H2 becomes a <details>/<summary>.
Top 4 sections (Header Controls, Chat, Sessions, Notes) open by default;
remaining sections (Backends, Talk, Files, Context, Shortcuts, API,
Planned) start collapsed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/app.js

@@ -884,6 +884,33 @@
         const helpBody  = document.getElementById('help-modal-body');
         const helpClose = document.getElementById('help-close-btn');
 
+        // Sections open by default — everything after "Notes" starts collapsed
+        const HELP_OPEN_SECTIONS = new Set(['Header Controls', 'Chat', 'Sessions', 'Notes']);
+
+        function makeCollapsible(container) {
+            const h2s = Array.from(container.querySelectorAll('h2'));
+            for (const h2 of h2s) {
+                const title = h2.textContent.trim();
+                const details = document.createElement('details');
+                if (HELP_OPEN_SECTIONS.has(title)) details.open = true;
+
+                const summary = document.createElement('summary');
+                summary.textContent = title;
+                details.appendChild(summary);
+
+                // Collect all following siblings until the next h2
+                const siblings = [];
+                let node = h2.nextSibling;
+                while (node && node.nodeName !== 'H2') {
+                    siblings.push(node);
+                    node = node.nextSibling;
+                }
+                for (const sib of siblings) details.appendChild(sib);
+
+                h2.parentNode.replaceChild(details, h2);
+            }
+        }
+
         async function openHelp() {
             helpBody.textContent = 'Loading…';
             helpModal.classList.add('open');
@@ -895,6 +922,7 @@
                 helpBody.querySelectorAll('a').forEach(a => {
                     a.target = '_blank'; a.rel = 'noopener noreferrer';
                 });
+                makeCollapsible(helpBody);
             } catch (err) {
                 helpBody.textContent = `Failed to load help: ${err.message}`;
             }