fix: show payable amount after discount on payment views

Hide the payable row when there is no discount, clamp create-form discounts, and cap notes length.

src/views/payments/PaymentDetailView.vue

@@ -33,7 +33,7 @@
               <span class="text-muted text-xs block mb-1">تخفیف</span>
               <span class="font-semibold text-orange-500 text-lg">{{ toPersianDigits((payment.discount || 0).toLocaleString()) }} تومان</span>
             </div>
-            <div>
+            <div v-if="payment.discount">
               <span class="text-muted text-xs block mb-1">مبلغ قابل پرداخت</span>
               <span class="font-bold text-color text-xl">{{ toPersianDigits(payableAmount.toLocaleString()) }} تومان</span>
             </div>
@@ -120,6 +120,7 @@
             v-model="trxForm.notes"
             rows="3"
             class="w-full text-sm"
+            maxlength="5000"
             placeholder="یادداشت داخلی درباره این تراکنش…"
           />
         </div>