feat: add Spanish landing page locale

[web-locale-contributor]

Summary

• Add a statically generated Spanish landing page at /es while keeping / as the stable English default.
• Localize visible first-party marketing copy and add an accessible, labelled language selector in both desktop and mobile navigation.
• Preserve in-page anchors when switching languages; do not add automatic locale or geo redirects.
• Add locale-aware titles, descriptions, canonical URLs, Open Graph metadata, hreflang alternates, and the Spanish entry to the sitemap.

Validation

• npx tsc --noEmit --incremental false
• NEXT_TELEMETRY_DISABLED=1 npm run build
• Responsive checks at 320 px, 768 px, and 1280 px

Closes #152

[vercel]

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