refactor(frontend): consolidate api utility proxies and error handlers

[victoriacheng15]

Summary

Consolidated multiple separate server-side API proxy handler files and error interceptor utilities into a single, unified module. This consolidation reduces file clutter, simplifies imports via the directory path resolver, and unifies the error formatting contract.

List of Changes

• Consolidated seven separate proxy request files and their corresponding vitest assertions into index.ts and index.test.ts under frontend/src/app/api/_utils/.
• Removed the deprecated legacy files and configured the exports directly under the consolidated entry point.

Verification

• npm run lint
• npm run test:frontend