Set the default quantity to one when adding new products to a sale

Sets the default `quantite` value to '1' in `NewSaleForm.tsx` and updates the input placeholder.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: e80fed01-7137-4997-b9af-dad57132d430
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/0715008c-7244-40f4-befc-26c014852236/e80fed01-7137-4997-b9af-dad57132d430/J2iBQMm

replit.md

@@ -332,6 +332,11 @@ Preferred communication style: Simple, everyday language.
 - ✅ **Validation Adjustment**: Photos no longer required for form submission compliance
 - ✅ **User Experience**: Added explanatory text to reduce confusion about photo requirements
 
+### Form Usability Improvements (January 19, 2025)
+- ✅ **Default Quantity**: Set product quantity field to default value of 1 for better user experience
+- ✅ **Streamlined Input**: Users no longer need to manually enter quantity for single item purchases
+- ✅ **Placeholder Update**: Changed placeholder text to reflect default value
+
 ### Docker Configuration & Cleanup (January 9, 2025)
 - ✅ Complete Docker setup optimized for external PostgreSQL database
 - ✅ Docker Compose configuration with RegisFlow application only