🚀 chore(MainPage): use constant for header text in user projects section

The changes in headerComponent remove unused imports and update the header text to use a constant instead of a hardcoded string. The constant is added in constants.tsx. The MainPage component is updated to use the constant for the header text in the user projects section. This improves code readability and maintainability.
🎨 style(headerComponent): remove unused imports and update header text to use constant
✨ feat(constants.tsx): add constant for header text in user projects section