Migrate Footer component from styled-components to plain CSS

Changes:
- Created Footer.css with all styled-components converted to plain CSS classes
- Updated Footer/index.tsx to use plain CSS with className instead of Styled.* components
- Migrated social media icons (Facebook, Twitter, Instagram, LinkedIn) to inline SVG
- Wrapped icon components with styled() for component selector compatibility
- Bound CSS variables from theme for dynamic values (colors, backgrounds)
- Preserved complex CSS grid layouts for responsive behavior
- Maintained vertical nav toolbar styling with clamp() calculation
- Used classNames package for className composition
- Preserved all functionality including:
  - Normal footer with mission, columns, social links
  - Portal footer variant
  - Contact dialog integration
  - Manage cookies links
  - Responsive breakpoints (37.5em, 37.6em, 60.1em)

Migration follows the patterns established in PLAIN_CSS_MIGRATION_GUIDE.md
and PLAIN_CSS_MIGRATION_LEARNINGS.md. All CSS variables are set before
spreading style props to allow overrides.

Note: Tests will be updated in a separate commit per task requirements.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: OpenStaxClaude