feat(frontend): Dark Mode Theme Engine — state refactor, animations, unit tests & screen reader support

[libby-coder]

Summary

This PR addresses all four Dark Mode Theme Engine UX enhancement issues:

• [Frontend] Refactor state logic for Dark Mode Theme Engine #947 — Refactored state logic from multiple useState calls to useReducer for atomic, predictable transitions. Fixed stale closure bug in the system media query listener using a themeRef. Initialization now runs in a single one-time useEffect, eliminating redundant listener re-registration on every theme change. Extracted resolveTheme and applyThemeToDom helpers for clarity.
• [Frontend] Implement framer-motion animations for Dark Mode Theme Engine #948 — Enhanced framer-motion animations on the ThemeToggle with spring physics, hover/tap scale effects, and smoother icon crossfade transitions between light, dark, and system states.
• [Frontend] Add unit tests for Dark Mode Theme Engine #949 — Added comprehensive unit tests for ThemeToggle covering loading skeleton, all icon states (sun/moon/system/error), ARIA attribute correctness, keyboard interaction, and screen reader announcement text.
• [Frontend] Improve screen reader support for Dark Mode Theme Engine #950 — Improved screen reader support with richer aria-label descriptions, aria-pressed to reflect dark-mode active state, aria-live polite region for theme-change announcements, and focus-visible ring styles for keyboard users.

Test plan

• Toggle cycles correctly: light → dark → system → light
• System theme respects prefers-color-scheme media query
• localStorage persists preference under the configured storageKey
• Error state shown and clears on retry
• Screen reader announces theme change on each toggle
• Animations play smoothly on theme switch (no jank on mobile)
• Focus ring visible when navigating by keyboard
• Mobile-responsive layout verified on Chrome/Safari/Firefox

Closes #947
Closes #948
Closes #949
Closes #950

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