refactor(BillingCard): update button styles for consistent sizing and improved layout

frontend/src/components/billing/BillingCard.jsx

@@ -172,6 +172,7 @@ export default function BillingCard({
               <button 
                 className="btn btn-outline-success" 
                 title="Записать платёж" 
+                style={{ minWidth: '6.5rem' }}
                 onClick={(e) => { e.stopPropagation(); onAddPayment?.(item); }}
               >
                 <IconCreditCard size={16} className="me-1" />
@@ -180,14 +181,16 @@ export default function BillingCard({
               <button 
                 className="btn btn-outline-secondary" 
                 title="Редактировать" 
+                style={{ minWidth: '6.5rem' }}
                 onClick={(e) => { e.stopPropagation(); onEdit?.(item); }}
               >
                 <IconEdit size={16} className="me-1" />
                 Изменить
               </button>
               <button 
-                className="btn btn-outline-danger" 
+                className="btn btn-outline-danger d-flex align-items-center justify-content-center" 
                 title="Удалить" 
+                style={{ minWidth: '6.5rem' }}
                 onClick={(e) => { e.stopPropagation(); onDelete?.(item); }}
               >
                 <IconTrash size={16} />