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/PaymentListView.vue

@@ -151,6 +151,7 @@
             v-model="createForm.notes"
             rows="3"
             class="w-full text-sm"
+            maxlength="5000"
             placeholder="یادداشت داخلی درباره این صورتحساب…"
           />
         </div>
@@ -267,6 +268,7 @@ const onClassesSelected = () => {
 
   if (!createForm.classes || createForm.classes.length === 0) {
     createForm.amount = 0;
+    createForm.discount = 0;
     return;
   }
   let totalFee = 0;
@@ -279,6 +281,9 @@ const onClassesSelected = () => {
   if (totalFee > 0) {
     createForm.amount = totalFee;
   }
+  if ((createForm.discount || 0) > (createForm.amount || 0)) {
+    createForm.discount = createForm.amount || 0;
+  }
 };
 
 const resetCreateForm = () => {