feat(discount): add «مدیریت تخفیف‌ها» to the desktop settings sidebar

The desktop settings sidebar (PurchaseSubscriptionSidebar) uses its own
NAV_ITEMS list, separate from SETTINGS_MENU; add the discounts entry there so
it shows in the settings menu on desktop too.

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

assets/admin/components/layout/PurchaseSubscriptionSidebar.tsx

@@ -19,6 +19,7 @@ const NAV_ITEMS: NavItem[] = [
   { key: 'payment',      label: 'مدیریت پرداخت',   to: '/admin/my-financial' },
   { key: 'appointment',  label: 'مدیریت نوبت دهی', to: '/admin/appointment-settings' },
   { key: 'insurance',    label: 'مدیریت بیمه',     to: '/admin/insurance-pricing' },
+  { key: 'discounts',    label: 'مدیریت تخفیف‌ها', to: '/admin/discounts' },
   { key: 'tags',         label: 'تگ ها',           to: '/admin/tags-settings' },
   { key: 'sms',          label: 'پیامک ها',        to: '/admin/sms-wallet' },
   { key: 'clinic',       label: 'مدیریت مطب',      to: '/admin/my-clinic' },