Improve UI/UX and add dark mode support

[RKS-ai-coder]

Summary

This pull request enhances the website's overall UI/UX while adding a complete light/dark theme experience.

Changes made

• Improved typography hierarchy and spacing
• Refined navbar styling
• Added a light/dark theme toggle (defaults to system theme on first visit)
• Updated theme-aware logo switching
• Improved sponsor cards and hover interactions for dark theme
• Updated event and achievement cards
• Improved responsiveness across multiple sections
• Applied consistent styling across light and dark modes

Notes

• The website follows the user's system theme by default on the first visit.
• Once a user selects Light or Dark mode, that preference is saved for future visits.
• No existing functionality was removed; these changes focus on UI improvements and theme support.