fix(profile): persist & expose birthday as a single Unix field

Birthday was never saved (hydrate forced null) and not returned in a form the
clients use. Now:
- hydrate stores the incoming `birthday` (Unix) into date_of_birth; `birthday`
  takes priority over a stray `date_of_birth: null` in the same payload so it
  can't be wiped.
- toArray exposes a single `birthday` (Unix) key — drop the duplicate
  `date_of_birth` output to avoid overlap.
- admin UserDetailPage reads profile.birthday (formatDate renders Jalali).

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

assets/admin/pages/UserDetailPage.tsx

@@ -36,7 +36,7 @@ interface UserProfileData {
   uuid: string;
   national_code: string | null;
   gender: string | null;
-  date_of_birth: number | null;
+  birthday: number | null;
   blood_type: string | null;
   marital_status: string | null;
   education: string | null;
@@ -539,7 +539,7 @@ export default function UserDetailPage() {
               <InfoCard icon={UserIcon} label="نام پدر" value={profile.fathers_name} />
               <InfoCard icon={ClipboardDocumentIcon} label="کد ملی" value={profile.national_code} mono copyable />
               <InfoCard icon={UserIcon} label="جنسیت" value={profile.gender ? GENDER_LABELS[profile.gender] ?? profile.gender : null} />
-              <InfoCard icon={CalendarIcon} label="تاریخ تولد" value={profile.date_of_birth ? formatDate(profile.date_of_birth as any) : null} />
+              <InfoCard icon={CalendarIcon} label="تاریخ تولد" value={profile.birthday ? formatDate(profile.birthday) : null} />
               <InfoCard icon={UserIcon} label="گروه خونی" value={profile.blood_type} />
               <InfoCard icon={UserIcon} label="وضعیت تأهل" value={profile.marital_status ? MARITAL_LABELS[profile.marital_status] ?? profile.marital_status : null} />
               <InfoCard icon={UserIcon} label="تحصیلات" value={profile.education} />