fix(theme): use hugo-coder default avatar and favicons

[lanthoor]

Summary

• Switch homepage avatar and favicon params in hugo.yaml to Hugo Coder's default image paths.
• Add Hugo Coder default avatar/favicon assets under static/images/ so the theme resolves to its expected defaults.
• Preserve previous custom favicons in version control by renaming them to static/img/favicon-old.png and static/img/favicon-old.ico.

Verification

• Built locally with hugo successfully.