[LungMAP] Add Open Graph and Twitter meta tags

[frano-m]

Summary

LungMAP currently has no Open Graph or Twitter meta tags, so shared links on Slack, Twitter, LinkedIn, etc. show no preview. Adding these tags will enable rich link previews when pages are shared.

Reference Implementation

• AnVIL Portal PR: feat: add open graph and twitter meta tags for rich link sharing (#3944) anvilproject/anvil-portal#3945 — OgMeta component pattern
• brc-analytics — original reference implementation (app/components/common/OgMeta/ogMeta.tsx)

Tags to Add

Tag	Value
og:title	"{pageTitle} - {appTitle}" or just "{appTitle}"
og:description	Page-specific description, falling back to site-wide default
og:image	1200x630 OG image with LungMAP branding
og:image:width	1200
og:image:height	630
og:site_name	"{appTitle}"
og:type	website
og:url	Canonical URL for the current page
twitter:card	summary_large_image
twitter:image	Same as og:image
meta description	Same as og:description

Implementation Approach

1. Create an OgMeta component (or reuse from findable-ui if upstreamed)
2. Render in pages/_app.tsx
3. Pass pageDescription through pageProps from pages that have frontmatter descriptions
4. Provide a default description for pages without one
5. Create a 1200x630 OG image with LungMAP branding
6. Ensure all pages have title and description metadata