feat: implement complete dashboard dark mode across all components

[Kruthica]

📝 Pull Request Description

Related Issue

Closes #349

Summary

This PR introduces a complete dark mode experience for the PrepPilot dashboard, allowing users to seamlessly switch between light and dark themes. The selected theme is persisted across sessions using local storage, while the initial theme respects the user's system preference (prefers-color-scheme). All major dashboard components have been updated to support both themes with accessible color contrast, ensuring a consistent and comfortable user experience.

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Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature
• ♻️ Refactoring
• 📝 Documentation update
• 🎨 UI/UX improvement
• 🔥 Other (please describe) ______

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How Has This Been Tested?

• Verified that the theme toggle switches instantly between light and dark modes.
• Confirmed the selected theme persists after page refresh and browser restart.
• Tested that the initial theme follows the system preference on first visit.
• Checked all major dashboard components (sidebar, cards, buttons, forms, modals, etc.) in both themes.
• Verified responsiveness and readability across different screen sizes.
• Confirmed there are no console errors or visual regressions.

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Screenshots

Before

After

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Checklist

• My code follows the project's guidelines
• I have tested my changes
• I have updated documentation where necessary
• I have linked the related issue
• My changes do not introduce new warnings or errors