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>

docs/api/resource-calendar.md

@@ -212,6 +212,10 @@
 > `GET /api/v1/national-holidays` برای ادمین هم کار می‌کند: او محیط کاری ندارد، پس
 > بخش `overrides` برایش خالی برمی‌گردد و فقط تقویم را می‌بیند.
 
+> **پنل:** صفحهٔ `/admin/national-holidays` (منو ← سیستم ← تعطیلات رسمی) تنها جای
+> ساخت و حذف است و فقط `ROLE_ADMIN` می‌بیندش. `/admin/holidays` مالِ محیط است و آنجا
+> فقط استثنا زده می‌شود، نه خودِ تعطیلی.
+
 ### `POST /api/v1/holiday-overrides`
 
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