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

components/layout/Header.tsx

@@ -18,7 +18,7 @@ export function Header() {
       className="sticky top-0 z-50 bg-canvas/80 backdrop-blur-sm border-b border-border"
     >
       <div className="max-w-4xl mx-auto px-6 py-4 flex items-center justify-between">
-        <Link href="/" className="font-sans text-xl font-extrabold tracking-tight text-ink hover:text-accent transition-colors">
+        <Link href="/" className="heading-sm text-ink hover:text-accent transition-colors">
           blog
         </Link>
         <div className="flex items-center gap-8">