fix(docs): improve mobile sidebar navigation

The sidebar had all categories expanded by default (collapsed: false),
which on mobile created a 60+ item flat list when opening the sidebar.
Reported by danny on Discord.

Changes:
- Set all top-level categories to collapsed: true (tap to expand)
- Enable autoCollapseCategories: true (accordion — opening one section
  closes others, prevents the overwhelming flat list)
- Enable hideable sidebar (swipe-to-dismiss on mobile)
- Add mobile CSS: larger touch targets (0.75rem padding), bolder
  category headers, visible subcategory indentation with left border,
  wider sidebar (85vw / 360px max), darker backdrop overlay

website/src/css/custom.css

@@ -199,6 +199,46 @@ pre.prism-code.language-ascii code {
   border: 1px solid rgba(255, 215, 0, 0.08);
 }
 
+/* ─── Mobile sidebar improvements ─────────────────────────────────────────── */
+
+/* Larger touch targets on mobile */
+@media (max-width: 996px) {
+  .menu__link {
+    padding: 0.6rem 0.75rem;
+    font-size: 0.95rem;
+  }
+
+  .menu__list-item-collapsible > .menu__link {
+    font-weight: 600;
+    font-size: 1rem;
+    padding: 0.75rem 0.75rem;
+    border-bottom: 1px solid rgba(255, 215, 0, 0.06);
+  }
+
+  /* Category caret — more visible */
+  .menu__caret::before {
+    background-size: 1.5rem 1.5rem;
+  }
+
+  /* Indent subcategories clearly */
+  .menu__list .menu__list {
+    padding-left: 0.75rem;
+    border-left: 1px solid rgba(255, 215, 0, 0.06);
+    margin-left: 0.5rem;
+  }
+
+  /* Sidebar overlay — slightly more opaque for readability */
+  .navbar-sidebar__backdrop {
+    background-color: rgba(0, 0, 0, 0.6);
+  }
+
+  /* Sidebar width on mobile — use more of the screen */
+  .navbar-sidebar {
+    width: 85vw;
+    max-width: 360px;
+  }
+}
+
 /* Hero banner for docs landing if needed */
 .hero--hermes {
   background: linear-gradient(135deg, #07070d 0%, #0f0f18 100%);