Fix marketing page layout regression

[smiggleworth]

Summary

• make vertical Container layout intent explicit across shared site wrappers
• restore stacked marketing heroes, home differentiation content, 404 content, and footer
• add browser regression coverage for every marketing route at mobile and desktop widths in light and dark modes

Verification

• npm run check
• 40/40 marketing layout browser cases
• 40-screenshot production-build visual audit (10 routes x 2 viewports x 2 themes)
• zero document overflow, clipped text, broken images, console errors, or page errors