Add default Open Graph and Twitter preview image

[AbdalrahmanWael]

Summary

When the website link is shared in places like Discord, X/Twitter, WhatsApp, or Slack, the preview should include a proper social card image instead of falling back to a text-only embed or inconsistent preview.

Current behavior

The base layout already sets og:title, og:description, og:type, og:url, and twitter:card, but og:image is only rendered when a page provides a thumbnail. This means pages without a thumbnail can be shared without a preview image.

Relevant code:

• src/_includes/layouts/base.njk

Proposed change

• Add a default site-wide social preview image for pages that do not define thumbnail
• Also emit matching Twitter image metadata (twitter:image)
• Ensure the image uses an absolute URL and recommended dimensions for large previews
• Allow blog posts to override the default image with their own thumbnail

Acceptance criteria

• Sharing the homepage shows a large preview card with image
• Sharing any blog post without a thumbnail still shows a default image
• Sharing a blog post with a thumbnail uses that image instead of the default
• Discord/X/Twitter preview validators pick up the image correctly