Modernize the user interface with a cleaner and more professional design

Refactors UI components to implement a modern, angular design with updated color scheme and styling in `NewSaleForm.tsx`, `Dashboard.tsx`, `Login.tsx` and `index.css`.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: dec54137-d5a2-4406-8ad4-5f2f9fb110bf
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/0715008c-7244-40f4-befc-26c014852236/6a7de82e-d1f1-4ccf-baa2-a2d2d7172ac9.jpg

replit.md

@@ -179,6 +179,11 @@ Preferred communication style: Simple, everyday language.
 - ✅ Made "Lieu de naissance" field mandatory in new sale form for regulatory compliance
 - ✅ Removed unnecessary header section from new sale form for cleaner interface
 - ✅ Created custom favicon with fireworks design representing RegisFlow application
+- ✅ Implemented modern UI template with angular design elements replacing rounded components
+- ✅ Updated color scheme to use darker, more professional tones with less vibrant gradients
+- ✅ Modified card layouts, buttons, and form elements to use rectangular shapes instead of rounded corners
+- ✅ Enhanced Dashboard with clean, professional navigation tabs using bottom borders instead of rounded active states
+- ✅ Simplified login page with modern, angular design maintaining accessibility and usability
 
 ### Database Backup & Recovery System (January 9, 2025)
 - ✅ Added complete database backup and restore functionality in administration panel