test(web): cover classNames utility

[imbhargav5]

Summary

• replace the placeholder Vitest assertion with focused coverage for the classNames helper
• cover joining multiple class names and omitting undefined entries

Verification

• pnpm --filter web test
• pnpm --filter web lint
• pnpm --filter web typecheck

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[imbhargav5]

CI did not pass, so this PR was not merged. The blocking check is Playwright e2e (24.x), failing in existing logged-in private item flows with Supabase permission denied for table private_items. Local verification for this test-only change passed: pnpm --filter web test, pnpm --filter web lint, and pnpm --filter web typecheck.