Launch Pricing and harden the public marketing site

[ShenzheZhu]

Closes #1021

Summary

• add a localized four-tier Pricing page with UI-only monthly/yearly switching
• move Docs into Resources and expose Pricing in desktop, mobile, and footer navigation
• add responsive mobile navigation, localized About/Research/Docs metadata and copy, and venue-neutral BibTeX citation
• add production metadata base, Open Graph/Twitter share image, robots rules, sitemap, and apex routing for Pricing
• align stale marketing design tokens and copy across all five supported locales

Verification

• pnpm --filter @humanly/frontend-user type-check
• pnpm build:all
• local /pricing returned HTTP 200
• local Pricing metadata checked in English, Simplified Chinese, Japanese, Spanish, and French
• staged diff secret scan and git diff --check

Boundaries

• no billing, checkout, entitlements, or quota enforcement
• Pro and Enterprise remain disabled and marked coming soon