test(frontend): add AppShell unit coverage

[deepsikha-dash]

Description

• Added direct unit coverage for AppShell.
• Added tests for mobile menu open/close behavior.
• Added tests to verify the mobile drawer closes when the route changes.
• Added tests for mobile drawer and bottom navigation rendering.
• Added behavior-focused assertions instead of render-only checks.

Related Issues

Closes #552

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?

cd frontend
npm run test

npx vitest run testing/unit/components/AppShell.test.tsx

Result:

Test Files  1 passed (1)
Tests       5 passed (5)

Verified:

• Mobile menu toggle behavior.
• Mobile drawer closes on route changes.
• Mobile drawer rendering.
• Bottom navigation rendering.

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]

Closing as superseded by #736, which covers the same AppShell test area with stronger route/user-event behavior and has now been merged. This PR also conflicts on the same new test file.