UI: Improve the Add Task model ui

[riya-k003]

📌 Description

Revamped the Add Task modal UI to create a cleaner, more modern, and visually polished user experience while maintaining the existing application theme and functionality.

🔗 Related Issue

Closes #4

🛠 Changes Made

• Improved modal layout, spacing, and visual hierarchy
• Added modern input styling, focus states, and responsive structure
• Added icons and required field indicators for better UX clarity
• Enhanced button styling and footer layout consistency
• Improved responsiveness across different screen sizes
• Integrated full dark/light mode support using CSS variables (--surface, --border, --text-main, --text-muted, --primary, --accent)
• Maintained complete existing form functionality

📸 Screenshots

Before

Dark mode

Light mode

✅ Checklist

• Code runs locally
• Followed project structure
• No console errors
• Properly tested changes
• Linked the issue

🚀 Notes for Reviewers

Focused on improving the overall visual experience and usability of the Add Task modal without changing the existing functionality or application theme. All validation logic, state management, scroll lock behaviour, and keyboard handling remain untouched. Only className strings and layout structure were modified. Dark/light mode now uses the app's existing CSS variables so the form automatically adapts when the theme toggles — no extra dark: Tailwind classes needed.