fix: use absolute URLs for OG/Twitter image meta tags

[aledx18]

Problem

og:image used a relative path, which social crawlers (including X)
can't resolve. plugin.html also had no OG/Twitter meta tags at all,
so sharing a plugin page showed nothing either.

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Fix

• index.html: use absolute URL for og:image, add og:url and
  explicit twitter:title/description/image
• plugin.html: add the same OG/Twitter meta tag block, reusing the
  existing marketplace preview image (no dedicated image was available)

Testing

• npm test passes
• Verified HTML renders correctly in browser

Future improvement

plugin.html currently reuses the marketplace preview image for all
plugins since pages are rendered client-side. Per-plugin OG images
would need server-side/prerendered pages — out of scope here.