fix: use decimal type for donation quantity to prevent precision loss

[Destroyer795]

• Changed donation quantity column from 'float' to 'decimal(10,2)' in Donation entity
• Fixes issue where entered quantities (e.g., 100) were showing as slightly less than entered value
• float type causes binary floating-point precision issues; decimal preserves exact decimal values
• Schema will auto-sync on next backend startup via TypeORM