- Complete rewrite of show.php: 6 KPI cards (original amount, down payment,
  remaining balance, interest, monthly payment, member's outstanding balance),
  progress bar showing paid vs total installments, financial breakdown panels
  (paid vs remaining split), grand total formula ribbon, overdue alerts,
  next-due-date card with countdown, enhanced schedule table with status
  highlighting and per-row pay forms
- Add recalculate() action to InstallmentController: corrects pending-only
  installment rows to flat simple-interest formula without touching paid rows
- Register POST /installments/{id}/recalculate route
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>