DOC: Jest warning shown after successful frontend tests (“Jest did not exit one second after the test run”)

### What's wrong with the existing documentation

Description:
When running PictoPy frontend tests, all tests pass successfully, but Jest still prints a warning about open async handles. This can confuse contributors into thinking something is wrong even when tests are green.

Environment:

OS: Windows 11

Frontend: React + Vite

Test runner: Jest

Steps to reproduce:

Go to frontend directory

Run:

npm test

Observed output:

Test Suites: 1 passed
Tests: 6 passed
Jest did not exit one second after the test run has completed.

Observed behavior:

All frontend tests pass

Application works correctly

Warning is printed after test completion

Expected behavior:

Either:

Jest exits cleanly, or

Documentation explains that this warning is harmless when tests pass

Why this matters:

New contributors may think frontend setup is broken

Creates unnecessary confusion during onboarding

Suggestion:

Document this warning in frontend testing docs

Or investigate async handles not being closed in tests

### Add ScreenShots

_No response_

### Record

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on this issue