Test: Improve UI test coverage and fix Jest setup

[japinderofficial-hub]

Description

Add Jest configuration, Babel preset, and test-only mocks/shims so frontend UI tests run reliably in this repo.
Add beginner-friendly component tests and convert several fragile tests to React Testing Library where practical.
Keep production code unchanged; all edits are test-only (configs, mocks, and tests).

Changes

• Added and updated test configuration:

• .babelrc with preset-react (runtime: automatic)
• Jest config and setup (jest.config.cjs, jest/setupTests.js)
• Jest mocks/shims for react-dom, queryString, and file/style imports
• UI stabilization via styledComponentsMock.js and jest/ui-mocks/*

• Added test coverage for:

• index, footer, tree
• Widget components (Alert, Button, Input, TextIcon)
• UI inputs (input, textarea)

• Migrated index.test.jsx to React Testing Library (RTL)

How to test

Install dependencies:
yarn install

Run the test suite:
yarn test --runInBand

Expected result:
All frontend tests execute; on my run the suite passed (12 suites, 18 tests).

Quick checks:
Confirm no production files were modified (only test/config files and new tests).
Run single suites while iterating, for example:
yarn test button.test.jsx

close #159

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[japinderofficial-hub]

All the tests passed successfully