feat: add custom 404 NotFound component and route (fixes #162)

[sathwikhbhat]

Description

• Add src/components/NotFound.jsx, which shows a friendly message, an illustration, and a link to home.
• Add wildcard route to render NotFound for unmatched routes.
• Improves UX versus the default browser or framework 404.
• Matches app styling and is responsive.

Related Issue

Closes #162

How to test

1. npm install
2. npm run dev
3. Navigate to /some/nonexistent/path and verify NotFound displays, link returns to home.

Screenshots

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (improvement to an existing feature)
• Breaking change (fix or feature that would cause existing functionality to change)
• Test (adds or updates tests only)
• Documentation (non-code change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• All new and existing tests passed

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[Copilot]

Pull Request Overview

This PR adds a custom 404 Not Found page to improve user experience when navigating to non-existent routes. The implementation includes a dedicated NotFound component with consistent app styling, responsive design, and helpful navigation options.

Key Changes:

• Created a new NotFound page component with themed styling and navigation
• Added a wildcard route to catch all unmatched paths

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
frontend/src/pages/NotFound.jsx	New 404 page component with header, error message, icon, and navigation links
frontend/src/App.jsx	Added wildcard route and imported NotFound component

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