feat: [lungmap] add open graph and twitter meta tags (#4798)

[frano-m]

Summary

• Replace LungMAP's OG image with a 512×512 render cropped from logoLungmap.png (lungs + globe icon, centered on white) — sharper than the upscaled-favicon version that landed in feat: add open graph and twitter meta tags for anvil-cmg (#4797) #4799.
• Add pageDescription frontmatter to LungMAP MDX pages (apis.mdx, metadata.mdx, privacy.mdx) so each renders a page-specific meta description and og:description instead of falling back to the site-level default.

Closes #4798

Notes

Stacked on #4799. Most of the OG infrastructure (component, site-level description, build-script copy, MDX frontmatter pipeline, entity title derivation) landed there as a byproduct of the AnVIL CMG work; this PR fills in the LungMAP-specific assets and copy on top.

Test plan

• npm run check-format, npx tsc --noEmit pass
• npm run build-dev:lungmap succeeds; rendered HTML on /apis, /metadata, /privacy shows page-specific meta description and og:description
• Verified public/og/og-image.png is the LungMAP icon after the lungmap build (no cross-contamination from previous anvil-cmg builds)
• Verify Slack / Twitter/X / LinkedIn / Facebook debugger previews after deploy

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