fix(mobile): birthday picker shows limited months when no date exists (#26407)

* ScrollDatePicker defaults maximumDate to DateTime.now(). When no birthday exists, the picker starts at today (Feb 2026) with max also Feb 2026 — so only Jan–Feb are available for the current year.

Fix applied: Added maximumDate: DateTime(DateTime.now().year, 12, 31) at person_edit_birthday_modal.widget.dart:93, allowing all 12 months to be selected while still preventing future-year birthdays.

* fix(mobile): initialize birthday picker to past date to prevent future birthdays

When no birthday exists, initialize to 30 years ago instead of today.
This allows all 12 months to be selectable while keeping maximumDate
as DateTime.now() to prevent future birthday selection.

Fixes issue where only current months were available due to maxDate constraint.

---------

Co-authored-by: socksprox <info@shadowfly.net>

mobile/lib/presentation/widgets/people/person_edit_birthday_modal.widget.dart

@@ -25,7 +25,7 @@ class _DriftPersonNameEditFormState extends ConsumerState<DriftPersonBirthdayEdi
   @override
   void initState() {
     super.initState();
-    _selectedDate = widget.person.birthDate ?? DateTime.now();
+    _selectedDate = widget.person.birthDate ?? DateTime(DateTime.now().year - 30, 1, 1);
   }
 
   void saveBirthday() async {
@@ -90,6 +90,7 @@ class _DriftPersonNameEditFormState extends ConsumerState<DriftPersonBirthdayEdi
             selectedDate: _selectedDate,
             locale: context.locale,
             minimumDate: DateTime(1800, 1, 1),
+            maximumDate: DateTime.now(),
             onDateTimeChanged: (DateTime value) {
               setState(() {
                 _selectedDate = value;