test: improve ThemeModeToggle coverage to 100%

[Hell1213]

Improved test coverage for ThemeModeToggle component from 70% to 100% across all metrics. Added test suite with 9 test cases covering component rendering, accessibility, theme switching functionality, and proper mocking of Radix UI dropdown components.

Contributes: #299

Checklist

• Ran npx prettier --write . (for formatting)
• Ran gofmt -w . (for Go backend)
• Ran npm test (for JS/TS testing)
• Added unit tests, if applicable
• Verified all tests pass
• Updated documentation, if needed

Updated documentation, if needed
Additional Notes
Part of frontend utils testing plan. Focused on meaningful behavior testing rather than coverage-only tests. No functional changes to existing component code.

Screenshot

[github-actions]

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[Hell1213]

Added ThemeModeToggle test coverage with proper Radix UI mocking to achieve 100% coverage across statements, branches, functions, and lines.

[its-me-abhishek]

@Hell1213 Thanks for working on this but it got fixed by #313 recently, closing for now!