- Rename og_highlight config to social_highlight
- Favicon now uses social_highlight as background color
- Center favicon text and use Open Sans font
- Add subtle text shadow to favicon
- Document favicon in README

- Add og_highlight config option for background color
- New layout: URL top-left, date top-right (posts only), title bottom-left
- Subtle gradient with noise for smooth appearance
- Light drop shadows for readability
- Format dates as "15 January, 2026"
- Bump version to 1.5.0

Add Docker section to README and update GitHub workflow to use
the dedicated blurt image instead of pandoc/core.

Support __BUILD_TIME__ (per-page) and __COMMIT_SHA__ placeholders
that are replaced during build. Document partials and placeholders
in README.