Add Open Graph image

[replakcan]

Description

Add Open Graph image support for better link previews.

Scope

• Create default Open Graph image
• Add metadata reference
• Add blog-specific OG image strategy if simple
• Test link preview metadata

Out of Scope

• Dynamic image generation requiring server runtime
• Complex social preview automation

Acceptance Criteria

• Default OG image exists
• Main pages reference OG image
• Blog pages use default or post-specific OG image
• Metadata builds successfully
• npm run build succeeds

Technical Notes

• Must remain compatible with static export.
• Avoid server-only dynamic OG generation.

Related

• Related to: Add SEO metadata