Narrow global theme transitions and add reduced-motion guard in GlobalStyles

[Copilot]

The global transition rule in GlobalStyles.js applied to broad structural/content tags (div, span, ul, li, etc.), causing unnecessary repaint/recalc surface area and unintended animations. This change scopes transitions to relevant theming/interactive elements and disables motion when users request reduced motion.

• Transition scope reduction

  • Removed broad content/container tags from the global transition selector.
  • Kept transitions on a smaller, intentional set used for theme/UI state changes (body, sectioning containers, and interactive controls).

• Accessibility: reduced motion support

  • Added a global prefers-reduced-motion: reduce override to disable transitions and animations (including pseudo-elements).

• Illustrative change

  @media (prefers-reduced-motion: reduce) {
    *,
    *::before,
    *::after {
      transition: none !important;
      animation: none !important;
    }
  }