fix(installments): guarantee new cheques exactly cover remaining installment amount

- genPreview: compute lastAmt = remaining - amt*(needed-1) to show exact
  total; display grand total (existing + new) when cheques already exist;
  highlight preview red if lastAmt would be ≤ 0
- generateCheques: guard against per-cheque amount too large (lastAmt ≤ 0)
  with clear Arabic error showing max allowed per-cheque value; last cheque
  always = remaining - amt*(needed-1) so existing+new always = PLAN_TOTAL
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Modules/Installments/Views/cheques.php

@@ -302,18 +302,31 @@ $coveragePct  = $planTotal > 0 ? min(100, round($uploadedTotal / $planTotal * 10
         if (!amt || amt <= 0 || REMAINING_COUNT <= 0) { box.style.display='none'; return; }
 
         // Count is driven by REMAINING_COUNT (cheques still needed), not by amount math
-        var needed   = REMAINING_COUNT;
-        var endNum   = NEXT_NUM_FOR_PLAN + needed - 1;
-        var lastAmt  = Math.round((PLAN_TOTAL - ALREADY_UPLOADED - amt * (needed - 1)) * 100) / 100;
-        var total    = amt * (needed - 1) + lastAmt;
+        var needed    = REMAINING_COUNT;
+        var remaining = Math.round((PLAN_TOTAL - ALREADY_UPLOADED) * 100) / 100;
+        var endNum    = NEXT_NUM_FOR_PLAN + needed - 1;
+        var lastAmt   = Math.round((remaining - amt * (needed - 1)) * 100) / 100;
+        // total of new cheques = amt*(needed-1) + lastAmt = exactly remaining amount
+        var newTotal  = Math.round((amt * (needed - 1) + lastAmt) * 100) / 100;
+        // grand total = existing + new = must equal PLAN_TOTAL
+        var grandTotal = Math.round((ALREADY_UPLOADED + newTotal) * 100) / 100;
 
         document.getElementById('genCount').textContent        = needed;
         document.getElementById('genAmtDisplay').textContent   = fmt(amt);
-        document.getElementById('genTotalDisplay').textContent = fmt(total);
+        document.getElementById('genTotalDisplay').textContent =
+            fmt(newTotal) + (ALREADY_COUNT > 0 ? ' (إجمالي كل الشيكات: ' + fmt(grandTotal) + ')' : '');
         document.getElementById('genNumRange').textContent     =
             (ALREADY_COUNT > 0)
-                ? 'تكملة من #' + NEXT_NUM_FOR_PLAN + ' ← #' + endNum + ' (الموجود: ' + ALREADY_COUNT + ' شيك)'
+                ? 'تكملة من #' + NEXT_NUM_FOR_PLAN + ' ← #' + endNum + ' (الموجود: ' + ALREADY_COUNT + ' شيك بقيمة ' + fmt(ALREADY_UPLOADED) + ')'
                 : '#' + NEXT_NUM_FOR_PLAN + ' ← #' + endNum;
+
+        if (lastAmt <= 0) {
+            box.style.background = '#FEF2F2';
+            box.style.border = '1px solid #FECACA';
+        } else {
+            box.style.background = '';
+            box.style.border = '';
+        }
         box.style.display = '';
     };
 
@@ -323,13 +336,25 @@ $coveragePct  = $planTotal > 0 ? min(100, round($uploadedTotal / $planTotal * 10
         var bank      = document.getElementById('gen_bank').value.trim();
         var startDate = document.getElementById('gen_start_date').value;
 
-        if (!amt || amt <= 0)   { alert('أدخل قيمة الشيك أولاً'); return; }
-        if (!bank)              { alert('أدخل اسم البنك أولاً');  return; }
-        if (!startDate)         { alert('أدخل تاريخ أول شيك');    return; }
+        if (!amt || amt <= 0)     { alert('أدخل قيمة الشيك أولاً'); return; }
+        if (!bank)                { alert('أدخل اسم البنك أولاً');  return; }
+        if (!startDate)           { alert('أدخل تاريخ أول شيك');    return; }
         if (REMAINING_COUNT <= 0) { alert('تم تسليم جميع الشيكات المطلوبة بالفعل'); return; }
 
-        var needed = REMAINING_COUNT;
-        var remaining = PLAN_TOTAL - ALREADY_UPLOADED;
+        var needed    = REMAINING_COUNT;
+        var remaining = Math.round((PLAN_TOTAL - ALREADY_UPLOADED) * 100) / 100;
+
+        // Guard: per-cheque amount * (needed-1) must leave a positive last cheque
+        var lastAmt = Math.round((remaining - amt * (needed - 1)) * 100) / 100;
+        if (lastAmt <= 0) {
+            var maxAmt = Math.round((remaining / needed) * 100) / 100;
+            alert(
+                'قيمة الشيك كبيرة جداً — الشيك الأخير سيكون صفراً أو سالباً.\n' +
+                'الحد الأقصى المسموح: ' + maxAmt.toFixed(2) + ' ج.م لكل شيك\n' +
+                '(المبلغ المتبقي ' + remaining.toFixed(2) + ' ج.م ÷ ' + needed + ' شيكات)'
+            );
+            return;
+        }
 
         rows = [];
         for (var i = 0; i < needed; i++) {
@@ -338,10 +363,8 @@ $coveragePct  = $planTotal > 0 ? min(100, round($uploadedTotal / $planTotal * 10
                 cheque_number: String(NEXT_NUM_FOR_PLAN + i),
                 bank_name:     bank,
                 cheque_date:   addMonths(startDate, i),
-                // Last cheque absorbs any rounding difference
-                cheque_amount: isLast
-                    ? Math.round((remaining - amt * (needed - 1)) * 100) / 100
-                    : amt,
+                // Last cheque absorbs rounding difference to guarantee total = PLAN_TOTAL
+                cheque_amount: isLast ? lastAmt : amt,
                 notes: '',
             });
         }