feat(discount): dedicated «مدیریت تخفیف‌ها» item in the settings menu

Add a standalone settings sidebar entry + route (/admin/discounts) rendering
DiscountsPage (DiscountTab inside SettingsLayout, doctor/clinic only), and
revert the temporary tab inside the subscription page. Cleaner, discoverable
placement.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/admin/components/layout/SettingsLayout.tsx

@@ -2,7 +2,7 @@ import React from 'react';
 import {
   CreditCardIcon, UserIcon, CalendarDaysIcon, BuildingOffice2Icon,
   BanknotesIcon, UsersIcon, ShieldCheckIcon,
-  TagIcon, ChatBubbleLeftRightIcon, UserCircleIcon, UserPlusIcon,
+  TagIcon, ChatBubbleLeftRightIcon, UserCircleIcon, UserPlusIcon, ReceiptPercentIcon,
 } from '@heroicons/react/24/outline';
 import PurchaseSubscriptionSidebar from './PurchaseSubscriptionSidebar';
 
@@ -28,6 +28,7 @@ export const SETTINGS_MENU: SettingsMenuItem[] = [
   { key: 'secretary',    label: 'مدیریت منشی',        icon: UsersIcon,                to: '/admin/my-secretaries' },
   { key: 'staff',        label: 'پرسنل',              icon: UserPlusIcon,             to: '/admin/staff' },
   { key: 'insurance',    label: 'مدیریت بیمه',        icon: ShieldCheckIcon,          to: '/admin/insurance-pricing' },
+  { key: 'discounts',    label: 'مدیریت تخفیف‌ها',    icon: ReceiptPercentIcon,       to: '/admin/discounts',            roles: ['doctor', 'clinic'] },
   { key: 'tags',         label: 'برچسب‌ها',           icon: TagIcon,                  to: '/admin/tags-settings' },
   { key: 'sms',          label: 'پیامک‌ها',           icon: ChatBubbleLeftRightIcon,  to: '/admin/sms-wallet' },
   { key: 'account',      label: 'حساب کاربری',        icon: UserCircleIcon,           to: '/admin/account-settings' },