feat: add table of contents with scroll spy

components/blog/TableOfContents.tsx

@@ -0,0 +1,68 @@
+"use client";
+
+import { useEffect, useState, useRef } from "react";
+
+interface TOCItem {
+  id: string;
+  text: string;
+  level: number;
+}
+
+export function TableOfContents() {
+  const [headings, setHeadings] = useState<TOCItem[]>([]);
+  const [activeId, setActiveId] = useState("");
+  const observerRef = useRef<IntersectionObserver | null>(null);
+
+  useEffect(() => {
+    const elements = Array.from(
+      document.querySelectorAll("article h2, article h3")
+    ) as HTMLElement[];
+
+    const parsed = elements.map((el) => ({
+      id: el.id,
+      text: el.textContent ?? "",
+      level: el.tagName === "H2" ? 2 : 3,
+    }));
+
+    setHeadings(parsed);
+
+    observerRef.current = new IntersectionObserver(
+      (entries) => {
+        entries.forEach((entry) => {
+          if (entry.isIntersecting) {
+            setActiveId(entry.target.id);
+          }
+        });
+      },
+      { rootMargin: "-20% 0px -70% 0px", threshold: 0 }
+    );
+
+    elements.forEach((el) => observerRef.current?.observe(el));
+
+    return () => observerRef.current?.disconnect();
+  }, []);
+
+  if (headings.length === 0) return null;
+
+  return (
+    <nav className="lg:sticky lg:top-8">
+      <h4 className="font-sans text-xs font-semibold uppercase tracking-wider text-ink-soft mb-3">
+        On this page
+      </h4>
+      <ul className="space-y-1">
+        {headings.map((heading) => (
+          <li key={heading.id}>
+            <a
+              href={`#${heading.id}`}
+              className={`block text-sm transition-colors ${
+                heading.level === 3 ? "pl-3 text-ink-soft" : "text-ink"
+              } ${activeId === heading.id ? "font-semibold text-accent" : ""}`}
+            >
+              {heading.text}
+            </a>
+          </li>
+        ))}
+      </ul>
+    </nav>
+  );
+}