feat(frontend): add custom 404 page for unknown routes

[Rohan-R07]

Description

This PR adds a custom 404 (Not Found) page for SecuScan and updates the application's routing to display the page when users navigate to an invalid or non-existent route.

Changes Made

• Added a dedicated NotFound page component.
• Added SecuScan branding and styling consistent with the existing dashboard design.
• Added a clear "Page Not Found" message and explanation.
• Added a "Return to Dashboard" button for easy navigation.
• Updated the catch-all route to render the custom 404 page instead of redirecting directly to the dashboard.
• Added/updated route tests to verify the new behavior.

Related Issues

Closes #1009

Type of Change

• New feature

How Has This Been Tested?

Manual Testing

1. Started the frontend application locally.
2. Navigated to valid routes and verified existing functionality remained unchanged.
3. Navigated to a non-existent route (e.g. /nonexistent-route).
4. Verified that the custom 404 page is displayed.
5. Verified that the "Return to Dashboard" button navigates correctly.
6. Verified the page renders correctly with the existing application theme.

Recording

Custom 404 page displayed when navigating to an invalid route.

Additional Verification

• Confirmed routing behavior for unknown URLs.
• Confirmed no impact on existing routes.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[Rohan-R07]

@utksh1 I have Implemented the custom 404 page and updated routing behavior for unknown routes. Added a screen recording for easier review.

Looking forward to your feedback. Thank you!