feat: add themed 404 NotFound page

[erbhuwan]

Feature: Themed 404 NotFound Page

Description

This PR adds a fully designed 404 NotFound page to improve user experience when navigating to non-existent routes.
The page is themed to match the Algorithm Visualizer aesthetics and supports both dark and light modes.

Features

• Full-page 404 with dark/light mode gradient backgrounds.
• Fun, programming-related quotes to keep the theme playful.
• Lucide icons (Code, Cpu, AlertCircle) floating in the background with subtle animations.
• Centered content with a responsive, styled "Go Back to Visualizer" button including an arrow icon.
• Icons are properly visible in both dark and light themes.
• Layout adjusted to respect wrapper padding (pt-20) without causing overflow.

Screenshots

How to Test

1. Run the application locally.
2. Navigate to a non-existent route, e.g., /non-existing-page.
3. Verify that the 404 page displays correctly in both dark and light modes.
4. Check that the "Go Back to Visualizer" button works and links back to /.