Fix AgentAction logo and favicon

[dinpd]

Closes #87

Summary

• add reusable 512×512 AgentAction PNG logo and 64×64 favicon assets
• use the PNG logo in the site header
• publish favicon and Apple touch icon metadata
• add rendered HTML and asset existence coverage

Acceptance criteria

• Rendered HTML includes a favicon reference.
• Favicon and logo assets are present in the public output.
• Logo preserves the dark circular mark with lime AA treatment.
• Existing build, lint, rendered HTML tests, and dependency audit pass.

Validation

• npm test ✅
• npm run lint ✅ (one existing framework advisory warning for static <img>)
• npm audit --omit=dev ✅ 0 vulnerabilities
• PNG inspection ✅ logo.png 512×512 RGBA; favicon.png 64×64 RGBA

Security

Static asset and metadata changes only; no credentials, permissions, data, or runtime trust boundaries changed.