refactor: move secretary and staff management from main nav to settings menu

Remove پرسنل (/admin/staff) and منشی ها (/admin/my-secretaries) from the
clinic and doctor main sidebars; surface them under the settings menu instead
(PurchaseSubscriptionSidebar + SETTINGS_MENU). Wrap StaffPage in SettingsLayout
so it renders inside the settings shell like MySecretariesPage already does.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/admin/components/layout/PurchaseSubscriptionSidebar.tsx

@@ -23,6 +23,8 @@ const NAV_ITEMS: NavItem[] = [
   { 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' },
+  { key: 'secretary',    label: 'مدیریت منشی',     to: '/admin/my-secretaries' },
+  { key: 'staff',        label: 'پرسنل',           to: '/admin/staff' },
   { key: 'account',      label: 'حساب کاربری',     to: '/admin/account-settings' },
   { key: 'security',     label: 'تنظیمات' },
 ];