Adds data-scroll-behavior="smooth" to <html> so Next disables the global
scroll-behavior during programmatic route-change scrolls while keeping
smooth behavior for user-driven anchor jumps. Silences the Next 15 warning.
Footer is fixed bottom-0 with h-footer (5rem mobile) / md:h-footer-wide
(4rem desktop). Body gets matching pb-footer/md:pb-footer-wide to
reserve space. Tokens registered in @theme inline as --spacing-footer*.
not-found.tsx renders oversized Fraunces heading with a back link.
Body gets flex flex-col min-h-screen so main can flex-1 to fill
available height without pushing the footer off screen.
Ilia Mashkov