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/style.css

@@ -898,6 +898,43 @@
             font-weight: 600;
         }
 
+        /* ── Help modal collapsible sections ────────────────────── */
+        #help-modal-body details {
+            border-bottom: 1px solid var(--border);
+        }
+        #help-modal-body details:last-of-type { border-bottom: none; }
+
+        #help-modal-body summary {
+            cursor: pointer;
+            padding: 10px 4px;
+            font-size: 1.0em;
+            font-weight: 600;
+            color: var(--accent);
+            display: flex;
+            align-items: center;
+            gap: 8px;
+            user-select: none;
+            list-style: none;
+        }
+        #help-modal-body summary::-webkit-details-marker { display: none; }
+
+        #help-modal-body summary::before {
+            content: '▶';
+            font-size: 0.6em;
+            opacity: 0.55;
+            transition: transform 0.15s;
+            flex-shrink: 0;
+        }
+        #help-modal-body details[open] > summary::before {
+            transform: rotate(90deg);
+        }
+        #help-modal-body summary:hover { color: var(--text); }
+
+        /* Content inside a details block gets a little left indent */
+        #help-modal-body details > *:not(summary) {
+            padding-left: 12px;
+        }
+
         /* ── Auth warning banner ─────────────────────────────────── */
         #auth-banner {
             display: none;