Products can now be configured to accept partial payment (deposit/عربون)
at checkout. When enabled, the cashier can collect a minimum deposit now
and the remaining balance stays on the invoice for later collection.

- Migration adds allows_partial_payment + minimum_deposit_percent to products
- Product form has toggle with configurable minimum % (10-90%)
- POS terminal shows deposit option when cart contains eligible products
- POSService creates full invoice but records only the deposit as payment
- POS transaction marked 'partially_paid' when deposit is used
- Products with deposit enabled show "عربون" badge in POS catalog
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>