All calculation sites now use: interest = remaining × (rate/100) × (months/12)
instead of diminishing-balance amortization.

Changes across all 6 sites:
- InstallmentCalculator: flat interest, equal monthly instalments
- PricingEngine: same formula
- RetroactiveMembershipService: same formula
- retroactive-wizard JS: updated preview + shows المبلغ المتبقي in summary
- Members/show.php: preview panel now includes months factor (was missing)
- Installments/create.php: added live المبلغ المتبقي preview panel
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>