fix: defer mobile ToC population to DOMContentLoaded

The inline script ran before the sidebar-toc element was in the DOM,
so querySelector returned null and no links were added.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

astro-site/src/components/Header.astro

@@ -92,29 +92,30 @@ const base = import.meta.env.BASE_URL;
   });
 
   // Populate "On this page" section from the sidebar ToC
-  const sidebarToc = document.querySelector(".sidebar-toc");
-  if (sidebarToc) {
+  document.addEventListener("DOMContentLoaded", () => {
+    const sidebarToc = document.querySelector(".sidebar-toc");
+    if (!sidebarToc) return;
     const topLinks = sidebarToc.querySelectorAll(":scope > ul > li");
-    if (topLinks.length > 0) {
-      const label = document.createElement("div");
-      label.className = "nav-toc-label";
-      label.textContent = "On this page";
-      siteTabs.appendChild(label);
+    if (topLinks.length === 0) return;
 
-      topLinks.forEach((li) => {
-        const source = li.querySelector(":scope > a, :scope > details > summary > a");
-        if (source) {
-          const link = document.createElement("a");
-          link.href = source.getAttribute("href");
-          link.textContent = source.textContent;
-          link.className = "nav-toc-link";
-          link.addEventListener("click", () => {
-            navToggle.setAttribute("aria-expanded", "false");
-            siteTabs.classList.remove("open");
-          });
-          siteTabs.appendChild(link);
-        }
-      });
-    }
-  }
+    const label = document.createElement("div");
+    label.className = "nav-toc-label";
+    label.textContent = "On this page";
+    siteTabs.appendChild(label);
+
+    topLinks.forEach((li) => {
+      const source = li.querySelector(":scope > a, :scope > details > summary > a");
+      if (source) {
+        const link = document.createElement("a");
+        link.href = source.getAttribute("href");
+        link.textContent = source.textContent;
+        link.className = "nav-toc-link";
+        link.addEventListener("click", () => {
+          navToggle.setAttribute("aria-expanded", "false");
+          siteTabs.classList.remove("open");
+        });
+        siteTabs.appendChild(link);
+      }
+    });
+  });
 </script>