Improve Invoice Generator UI for Better User Experience

The current invoice generator UI works correctly, but the overall design and user experience can be improved.

This issue focuses on enhancing the interface to make it more clean, modern, and user-friendly. The goal is to improve readability, layout structure, and visual consistency without changing the core functionality.

Proposed Improvements:

Improve layout spacing and alignment

Enhance typography and font consistency

Update colors for a more professional look

Improve form input styling

Make the UI more responsive across screen sizes

Improve invoice preview design

These changes will help users create invoices more easily and provide a better visual experience.