Build theme settings page (dark/light/system) with persistence

[Nemenwq]

I've successfully built the theme settings page with full persistence and system preference support. Here's what was implemented:

✅ Key Features Delivered:
Three Theme Options: Light, Dark, and System (follows OS preference)
Settings Page: Added "Appearance" section to /dashboard/settings/preferences
Persistence: localStorage saves theme choice, survives browser refresh
No Flash: CSS transitions ≤ 200ms prevent theme flicker on load
System Integration: Automatically syncs with OS dark/light mode preference
Immediate Updates: Theme changes apply instantly without page reload
🎨 Design Compliance:
Theme buttons meet 44px minimum height requirement
Smooth transitions prevent visual jarring
Uses existing WCAG AA compliant color tokens
Consistent with existing settings page design
🔧 Technical Implementation:
ThemeProvider: New context provider managing theme state with system preference detection
Updated Components: ThemeToggle now uses centralized theme management
CSS Transitions: Added smooth color transitions to prevent flash
TypeScript Support: Full type safety with ThemeMode enum
The theme settings are now live and ready for use. Users can access them through Dashboard → Settings → Preferences → Appearance section.

Closes #79

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[Abidoyesimze]

Kindly fix conflict @Nemenwq