fix: restore docs footer contrast in light mode

[abrichr]

What changed

• explicitly color Material's inner copyright wrapper in the light palette
• explicitly color footer links and social icons, including hover and focus states
• keep the existing dark palette unchanged

Why

The live footer container had the intended warm light background and dark
foreground, but MkDocs Material applies semi-transparent white directly to
.md-copyright__highlight and footer links. Those child declarations overrode
the inherited brand color, producing approximately 1.01:1 white-on-white
contrast in light/system mode.

The fixed light palette computes to 7.92:1 for copyright text and 5.98:1 for
links/icons on the footer background, exceeding WCAG AA for normal text. Dark
visible text remains 9.39:1.

Validation

• uv run pytest tests/ -q — 76 passed
• uv run python scripts/validate_docs.py — passed
• uv run mkdocs build --strict — passed
• headless Chromium screenshots and computed-style checks at 1440×900 and
  390×844 for light, dark, and no-preference/system-default palettes
• git diff --check