feat: add dark mode toggle with localStorage and prefers-color-scheme…

[Pidoko257]

Closes #244
… support

• Add CSS variables for light/dark themes to index.css
• Wire DarkModeToggle into nav in App.jsx
• Toggle persists preference in localStorage, defaults to prefers-color-scheme
• Accessible aria-label: 'Switch to dark/light mode'

Description

Please include a summary of the changes and related context.

Closes #(issue number)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Changes Made

Testing

Please describe the tests you ran and how to reproduce them:

• Unit tests pass
• Integration tests pass
• Manual testing completed

Test Coverage:

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests passed locally with my changes
• Any dependent changes have been merged and published

Screenshots (if applicable)

If your changes include UI modifications, please add screenshots here.

Additional Notes

Any additional information that reviewers should know.

[dev-fatima-24]

@Pidoko257 you're pushing from main, pls push from branch

[Pidoko257]

I have implemented the correction, please check recent pull request. thanks!