Add a landing page in front of the assessment

[ksale001]

Summary

• Adds a marketing landing page at / (nav + 8 sections + footer, the heartbeat "beat" animation). The assessment app moves to /assess; share pages (/[code]) and /methodology are unchanged.
• Light theme is a warm, ecosystem-neutral cream palette, deliberately distinct from the app's dark teal. It's fully scoped under .vb-landing so its token overrides never leak into the assessment app.
• Dark mode maps the landing tokens to the app's existing dark-teal palette, so the two surfaces feel cohesive in dark (they diverge only in light). Reuses the existing next-themes setup and the same ThemeToggle, now in the landing nav.
• Reuses the shared Pizza component (added a backward-compatible glowOpacity prop so the slices read softer on cream).
• Renames the "Infrastructure" slice to "Provider" across the rubric (second commit) for consistency with the methodology and the landing copy.

Notes

• valOS links point at the VALOS spec; methodology links stay internal.
• The 729 OG images regenerate at build time (build:og), so no committed images needed updating.
• Follow-up: a separate PR will expand this light theme to the assessment app itself.

How to test it

• yarn test, yarn lint, and yarn build pass (ran locally; full static export builds all routes)
• / shows the landing; "Assess your validator" and the in-app links go to /assess/
• Toggle dark mode in the nav — landing flips to the app's dark palette; the choice persists and follows you to /assess/
• Pizza, share pages, and methodology all render with the "Provider" label