Add Jest tests for React frontend

[martinscoding]

Adds Jest and React Testing Library setup for the frontend.

Changes:

• Adds Jest configuration for the Vite React frontend
• Adds setupTests.js and file mocks
• Adds frontend tests for loading, creating and validating tasks
• Adds TEST_RESULTS.md with documented test results
• Adds Git workflow documentation

Validation:

• npm test -- --runInBand
• npm run lint
• npm run build

[DerDexBot]

Welldone