header logo + legal pages: fit-to-header + top inset

- Header link sizes to 70% of the header height (aspect-square) so it
  fits at all breakpoints instead of overflowing the 100px default
  height. Was hard-pinned to 120×120 which clipped on small headers.
- LegalPage adds pt-height-header so content starts below the fixed
  header instead of being covered by it (the Landing route stays
  full-bleed intentionally).

src/components/LegalPage.tsx

@@ -3,7 +3,7 @@ import Section from "./Section";
 
 const LegalPage: FC<{ children: ReactNode }> = ({ children }) => {
 	return (
-		<Section>
+		<Section className="pt-height-header">
 			<article className="markdown">{children}</article>
 		</Section>
 	);