fix: add matching favicon for sidebar branding consistency (#2363)

[aaniya22]

Description

Adds a favicon matching the sidebar's SecuScan shield branding. Previously index.html had no favicon link at all, so the browser tab displayed a generic default icon while the sidebar rendered a red glowing shield via a Material Symbols font icon. Added an SVG favicon (red shield, same color and glow as the sidebar) and linked it in the document head.

Related Issues

Fixes #2363

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Ran npm run dev in frontend/, opened 127.0.0.1:5173, hard-refreshed (favicons are cached by the browser), and confirmed the tab icon now shows the red shield, matching the sidebar logo. Tested in Chrome.

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.

[utksh1]

Reviewed the favicon addition and the existing required checks are green. Approved for squash merge.

[utksh1]

The favicon change is approved from a code-review perspective. The current branch is blocked by the repository-required frontend audit, which reports a pre-existing high-severity undici vulnerability; I am leaving the security gate intact rather than using an admin merge.