fix: unify heading typography with utility classes + add h4-h6

- app/page.tsx: h2 uses heading-md (was text-2xl font-semibold)
- app/not-found.tsx: h1 uses heading-xl (was text-4xl font-bold)
- Header.tsx: logo uses heading-sm (was text-xl font-extrabold)
- mdx-components.tsx: add h4/h5/h6 component overrides
- Remove all ad-hoc font-sans from headings

app/page.tsx

@@ -17,7 +17,7 @@ export default async function HomePage() {
         </p>
       </header>
       <section>
-        <h2 className="font-sans text-2xl font-semibold tracking-tight text-ink mb-8">
+        <h2 className="heading-md text-ink mb-8">
           Latest Posts
         </h2>
         <PostList posts={posts} />