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/not-found.tsx

@@ -7,7 +7,7 @@ export default function NotFound() {
     <main className="min-h-screen flex items-center justify-center px-6">
       <div className="text-center">
         <p className="font-mono text-sm text-ink-soft mb-4">404</p>
-        <h1 className="font-sans text-4xl font-bold tracking-tight text-ink mb-4">Page not found</h1>
+        <h1 className="heading-xl text-ink mt-0 mb-4">Page not found</h1>
         <p className="text-ink-soft mb-8">The page you{`'`}re looking for doesn{`'`}t exist.</p>
         <Link href="/" className="inline-block rounded-lg bg-ink px-6 py-3 text-sm font-medium text-canvas hover:opacity-80 transition-opacity">
           Go home