fix(installments): correct last-row rounding in recalculate action

The last pending row was absorbing full rounding relative to
(pendingCount-1), but the plan may have 30 paid rows already.
Now computes the pending interest pool (total - paid interest),
distributes flat per row, and absorbs only the pending rounding
on the final pending row.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Modules/Installments/Controllers/InstallmentController.php

@@ -229,17 +229,29 @@ class InstallmentController extends Controller
         $runningBalance = bcsub($remaining, (string) ($paidPrincipalSum['s'] ?? '0'), 2);
         if (bccomp($runningBalance, '0', 2) < 0) $runningBalance = '0.00';
 
+        // Compute how much interest is already baked into paid rows
+        $paidInterestSum = $db->selectOne(
+            "SELECT COALESCE(SUM(interest), 0) as s FROM installment_schedule
+             WHERE installment_plan_id = ? AND status = 'paid'",
+            [(int) $id]
+        );
+        $interestAlreadyPaid = (string) ($paidInterestSum['s'] ?? '0');
+        $pendingInterestPool = bcsub($correctTotalInterest, $interestAlreadyPaid, 2);
+        if (bccomp($pendingInterestPool, '0', 2) < 0) $pendingInterestPool = '0.00';
+
         $db->beginTransaction();
         try {
             $pendingCount = count($pendingRows);
+            $distributedInterest = '0.00';
             foreach ($pendingRows as $idx => $row) {
                 $isLast    = ($idx === $pendingCount - 1);
                 $principal = $isLast ? $runningBalance : $flatPrincipal;
-                // Last row absorbs rounding in interest too
+                // Last pending row absorbs all remaining interest rounding
                 $interest  = $isLast
-                    ? bcsub($correctTotalInterest, bcmul($flatInterest, (string) ($pendingCount - 1), 2), 2)
+                    ? bcsub($pendingInterestPool, $distributedInterest, 2)
                     : $flatInterest;
                 if (bccomp($interest, '0', 2) < 0) $interest = '0.00';
+                $distributedInterest = bcadd($distributedInterest, $interest, 2);
                 $amount = bcadd($principal, $interest, 2);
 
                 $runningBalance = bcsub($runningBalance, $principal, 2);