feat: implement persistent light/dark theme support

[Oyinkans0la12]

Closes #365

Closes #366

Summary

Adds application-wide light/dark theme switching with persistent user preferences and system theme fallback support.

Changes

• Added global theme management
• Implemented light/dark mode toggle UI
• Persisted theme preference across sessions
• Added automatic system theme detection fallback
• Updated app styling to support both themes consistently
• Added hydration-safe theme initialization

Features

• Toggle between:

  • Light theme
  • Dark theme

• Preference persistence using local storage/cookies

• Default fallback to system preference when no explicit selection exists

• Smooth theme transitions without flash issues

Technical Details

• Centralized theme provider/context
• Theme state shared across application
• SSR/hydration-safe initialization where applicable
• Accessible toggle controls and labels added

Testing

Added/updated tests for:

• theme toggle behavior
• persisted preference loading
• system preference fallback
• UI rendering in both themes
• hydration-safe initialization

Notes

• Existing styling preserved where possible
• Theme tokens/classes structured for future scalability

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