test: add Vitest coverage for interface utilities

[wyattrspeir-coder]

Summary

Adds focused Vitest coverage for plugins/interface/utils/index.tsx without changing production behavior.

Covered behavior

• cn() combines conditional classes correctly.
• cn() resolves conflicting Tailwind utilities using the last applicable class.
• getAssetImportTagsFromManifest() includes the current page's JS entry.
• Shared vendor and components chunks are included.
• CSS assets from the Vite manifest are included.
• Unrelated page scripts are excluded.
• Calling the helper without a current page still includes shared assets.
• Expectations are derived from manifest.json, avoiding brittle hard-coded Vite hashes.

Verification

Local simulation performed against the current plugins/interface/utils/index.tsx logic and committed public/.vite/manifest.json:

• strict TypeScript compatibility simulation: PASS
• executable behavior simulation: PASS

The upstream repository CI should still be treated as authoritative because this execution environment could not download the repository dependencies directly.

Demo

Short visual walkthrough attached below. It clearly distinguishes the local simulation from authoritative upstream CI.

/claim #71

starbasedb-bounty-71-demo.mp4