feat: landing page UX improvements — mock result card, crosshatch hero, teaser centering, nav home link

ux: improve landing — fix step visuals, add try-it teaser input, expand footer with nav links

feat: add landing page — hero, modes, how-it-works, stats, CTA band

fix: cold start 502, favicon, verify state persistence

feat(frontend): apply frontend-design, web-design-guidelines, architect-review skills