Add test environment, configuration, and unit tests for utils

[akib22]

Summary

This PR adds a testing environment and initial unit tests for the repository’s utility functions to improve reliability and make it easier for contributors to validate changes.

• Add test configuration and tooling (test framework, setup files, and npm/yarn test scripts).
• Add unit tests for utility functions (cache, fullscreen, utils).

How to run the tests locally

Install dependencies: npm install (or yarn)
Run test: npm run test
Run test with UI: npm run test:ui
Generate coverage: npm run test:coverage

Coverage Report

Request for feedback

• Do you prefer a different testing framework or directory structure for tests?
• I'd appreciate any feedback on the testing strategy and am eager to incorporate your suggestions in upcoming test files

Related to

Issue #635

[vercel]

@akib22 is attempting to deploy a commit to the dottle's projects Team on Vercel.

A member of the Team first needs to authorize it.