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>

src/UserProfile/Entity/UserProfile.php

@@ -160,7 +160,7 @@ class UserProfile
             'national_code'               => $this->nationalCode,
             'national_code_approved'      => $this->nationalCodeApproved,
             'gender'                      => $this->gender,
-            'date_of_birth'               => $this->dateOfBirth,
+            'birthday'                    => $this->dateOfBirth,
             'blood_type'                  => $this->bloodType,
             'marital_status'              => $this->maritalStatus,
             'education'                   => $this->education,