feat(admin): a page for the official holiday calendar

The three admin endpoints shipped without anywhere to call them from, so the
person who is supposed to maintain the national calendar could only do it
with curl or a console command. That is not "the director can register the
year's holidays".

/admin/national-holidays is ROLE_ADMIN only and sits under the System group
in the admin nav. The year lives in the query string, so back and refresh
return to the year being edited.

Editing takes only the title: `date` is the unique key, so moving a holiday
is really deleting one and creating another, and the form says so rather than
silently creating a duplicate. The date field is the shared Jalali picker,
which speaks Gregorian, so the page converts before POSTing the jalali_date
the API expects — and shows the converted value under the field so the user
can see what will be stored.

Deleting warns that the day leaves every clinic's calendar, because it does.

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

assets/admin/App.tsx

@@ -83,6 +83,7 @@ import SkillsPage from './pages/SkillsPage';
 import ResourcePoolsPage from './pages/ResourcePoolsPage';
 import ResourceDetailPage from './pages/ResourceDetailPage';
 import HolidaysSettingsPage from './pages/HolidaysSettingsPage';
+import NationalHolidaysPage from './pages/NationalHolidaysPage';
 import PatientRecordFormPage from './pages/PatientRecordFormPage';
 import PatientDetailPage from './pages/PatientDetailPage';
 import PaymentSuccessPage from './pages/PaymentSuccessPage';
@@ -300,6 +301,8 @@ export default function App() {
         <Route path="resources/:resourceUuid" element={<RoleRoute roles={['doctor', 'clinic', 'secretary']} blockClinicScope permission={['appointment_settings', 'view']}><ResourceDetailPage /></RoleRoute>} />
         {/* تقویم منبع در تب «ساعات کاری» همان صفحه حل شده؛ لینک‌های قدیمی نباید بشکنند. */}
         <Route path="resources/:resourceUuid/calendar" element={<ResourceCalendarRedirect />} />
+        {/* تقویم رسمی کشور — فقط مدیر سیستم. /holidays مالِ محیط است و فقط استثنا می‌زند. */}
+        <Route path="national-holidays" element={<RoleRoute roles={['admin']}><NationalHolidaysPage /></RoleRoute>} />
         <Route path="holidays" element={<RoleRoute roles={['doctor', 'clinic', 'secretary']} blockClinicScope permission={['appointment_settings', 'view']}><HolidaysSettingsPage /></RoleRoute>} />
         <Route path="sms-wallet" element={<RoleRoute roles={['doctor', 'clinic', 'secretary']} blockClinicScope permission={['sms', 'view']}><SmsWalletPage /></RoleRoute>} />
         <Route path="my-secretaries" element={<RoleRoute roles={['doctor', 'clinic']} blockClinicScope><MySecretariesPage /></RoleRoute>} />