[FIX] Improve dark mode text contrast on landing and shared components (Fixes #11)

[debangshu919]

Summary

This PR addresses unreadable text in dark mode where content blended into the background. It enables Tailwind's class-based dark mode and updates landing page typography and cards to use appropriate dark: color tokens with sufficient contrast.

Linked Issue

resolves #11

Changes

• Enable Tailwind dark mode class strategy
  • frontend/tailwind.config.js: darkMode: 'class'
• Global background/text defaults for dark mode
  • frontend/src/index.css: add .dark body { background-color: #111827; color: #e5e7eb; }
• Landing page contrast fixes
  • frontend/src/pages/Home.jsx:
    • Headings and paragraphs: add dark:text-white and dark:text-gray-300
    • Feature cards: dark:bg-gray-800, dark:text-white, dark:text-gray-300
    • Category cards: dark:bg-gray-800, headings/desc updated for dark
    • Algorithm tags: dark:bg-gray-700, dark:text-gray-200

Testing

• Manual checks across routes with theme toggle:
  • All texts are readable in dark mode.
  • Light mode remains visually consistent.

Screenshots

Checklist

• Dark mode enabled via class strategy
• Landing page text meets contrast guidelines
• Changes scoped and tested locally