- Changed variable `img_cdn` to `cdn` in site configuration file.
- Changed the variable defining the relative path of the image in the post from `img_url` to `media_subpath`
- Add configuration field `social_preview_image` that sets the site-wide default social preview image.
For pages that do not have `page.image` set in font matter, the seo tag will use the image specified by `site.social_preview_image` as the Open Graph image.
- Refactored the generation of image URLs to reduce redundant code and enhance fault tolerance for missing or repeated slash `/` when defining image paths.
