feat(marketing): add design-system foundation and component-preview brand pages

Summary

This MR adds the foundation of the marketing design system and documents it in the component preview app. It does not change any marketing page yet, on purpose.

What it adds:

• Shared style tokens for spacing, layout widths, section rhythm, and motion, plus mono and success colors.
• Reusable building-block classes: card, surface, CTA row, divider row, and prev/next pager, along with scroll-reveal hooks.
• The marketing type scale (display, title, and eyebrow tiers) and a larger icon subset.
• Shared components: banner, page shell, section header, and code card.
• Eight new Brand pages in the component preview that show how to use all of the above.

Everything here is additive. The existing page styles (old hero, centered block, prose blocks) are left exactly as they were, so this is safe to merge on its own without touching the live site. The real page redesigns come in follow-up MRs that build on these pieces.

Testing

• Built the marketing app in production mode: passes, all routes prerender, no new warnings.
• Ran the component preview locally and clicked through all eight Brand pages (Layout, Buttons and CTAs, Cards and surfaces, Motion, Section header, Code card, Page shell, Banner). They render the real shared components with no console errors.
• Ran the marketing site locally and confirmed the existing pages look unchanged (home renders with no errors).

AI disclosure: Claude Code. Drafted with AI assistance, reviewed and tested by hand.

• I am respecting Sentry's proprietary Business Source License (BSL) by implementing code in a clean room environment. I have not read any BSL Sentry code. It is OK to read MIT licensed code.