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

mdx-components.tsx

@@ -19,6 +19,15 @@ export function getMDXComponents(components: MDXComponents): MDXComponents {
         {props.children}
       </h3>
     ),
+    h4: ({ children, ...props }) => (
+      <h4 className="heading-sm text-ink mt-8 mb-3" {...props}>{children}</h4>
+    ),
+    h5: ({ children, ...props }) => (
+      <h5 className="text-base font-semibold text-ink mt-6 mb-2" {...props}>{children}</h5>
+    ),
+    h6: ({ children, ...props }) => (
+      <h6 className="text-sm font-medium uppercase tracking-wider text-ink-soft mt-6 mb-2" {...props}>{children}</h6>
+    ),
     p: ({ children }) => (
       <p className="leading-7 [&:not(:first-child)]:mt-6 text-ink">
         {children}