test: add unit tests for frontend api client wrapper

[Kishalll]

🔗 Related Issue

Closes #450

📝 What does this PR do?

Adds comprehensive unit tests for the frontend API client (frontend/src/lib/api.ts).

Specifically, this PR introduces frontend/src/lib/__tests__/api.test.ts which successfully mocks
global.fetch and localStorage to thoroughly verify:
- Proper formatting and injection of Authorization Bearer tokens.
- Parameter and body serialization for both JSON requests and FormData payloads (without manually
overriding the Content-Type boundary).
- Error handlers correctly identifying and throwing connection errors and custom server errors.
- Automatic token refresh logic properly catching 401s and retrying the failed request with newly
provisioned tokens.

🗂️ Type of Change

• 🐛 Bug fix

• ✨ New feature

• 🔧 Refactor / code cleanup

• 📝 Documentation update

• 🎨 UI / styling change

• ⚙️ CI / tooling / config change

• 🧪 Tests

  ---

  🧪 How was this tested?

  • Ran the backend locally (uvicorn app.main:app --reload)
  • Ran the frontend locally (npm run dev inside frontend/)
  • Tested the affected API endpoints manually
  • Added / updated tests

  ---

  📸 Screenshots (if UI change)

  N/A

  ---

  ⚠️ Anything to flag for reviewers?

  No major flags! I leveraged the jsdom Vitest environment previously introduced in dev to properly
  simulate browser objects like localStorage and FormData in the testing framework.

  ---

  ✅ Self-Review Checklist

  • My branch is based on dev, not main
  • I have not added any secrets / API keys
  • I have not modified main branch or any HuggingFace deployment config
  • My code follows the existing style (no unnecessary formatting changes)
  • I have updated relevant docs / comments if needed

[Kishalll]

@param20h resolved conflicts pls review