feat(theme-toggle): add sticky bottom action bar on scroll

[MJ-RWA]

Summary

Implements a sticky bottom action bar for the ThemeToggle page that appears while scrolling, making primary actions easily accessible without requiring users to scroll back to the original action area.

Changes Made

• Added a sticky bottom action bar to src/pages/ThemeToggle.tsx.
• Configured the action bar to appear after the user scrolls beyond the primary action section.
• Reused existing primary action handlers to avoid duplicate logic.
• Ensured responsive behavior across mobile, tablet, and desktop layouts.
• Applied existing design tokens for consistent light and dark mode styling.
• Added smooth visibility transitions while respecting prefers-reduced-motion.
• Implemented accessibility improvements, including keyboard navigation, visible focus states, semantic markup, and appropriate ARIA attributes.
• Added focused tests covering sticky bar visibility, scroll behavior, action functionality, and accessibility.
• Updated documentation to describe the new scrolling behavior and confirmed that there are no public API changes.

Testing

• Added focused unit/component tests for:

  • Initial hidden state.
  • Visibility after scrolling.
  • Hiding when returning to the top.
  • Primary action functionality.
  • Accessibility attributes and keyboard interaction.

• Verified linting and existing test suite pass.

Visible Changes

• ThemeToggle now displays a sticky bottom action bar with primary actions while users scroll through the page, improving usability on both desktop and mobile devices.

Closes: #731

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[greatest0fallt1me]

Merging this one — appreciate the thorough approach!