Center images when the footer is hidden

[infeng]

Summary

• treat the effective footer height as zero when noFooter is enabled
• keep footerless images centered during initial layout, reset, and resize
• extend the noFooter test to assert that no vertical space is reserved

Related to #130. The issue is intentionally left open pending maintainer confirmation.

Root cause

The footer DOM was hidden, but layout calculations still subtracted the fixed 84px footer height. This moved the image center 42px above the viewport center.

Validation

• npm run verify
• registry scan: only https://registry.npmjs.org/
• Chromium 149.0.7827.55: before fix center offset -42px; after fix 0px
• Chromium resize check: center offset remains 0px; no page errors