feat: add canonical URLs and meta tags for SEO

[Vaishnav88sk]

Description

Without canonical URLs, search engines may index duplicate content from different URL variations (with/without trailing slashes, different case, etc.). This dilutes page authority and can hurt rankings.

SEO enhancement requested in issue #209. Canonical tags are essential for any professional website to ensure proper search engine indexing.

This PR adds to the head.html partial:

• Canonical URL link tags pointing to the canonical permalink
• Meta description with fallback to summary/site description
• Meta keywords support (for engines that still use them)
• Author meta tag for bylines

How will this change help?

• Consolidate link equity to preferred URLs
• Prevent duplicate content issues
• Provide explicit metadata for search engines
• Improve keyword targeting with proper description and keywords meta

Related issue(s)

#209