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

[socksprox]

Description

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: 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.

How Has This Been Tested?

• Manually tested adding a new birthday (no prior date) — all 12 months now selectable
• Manually tested editing an existing birthday — still works correctly

Checklist:

• I have carefully read CONTRIBUTING.md
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR.
• I have confirmed that any new dependencies are strictly necessary.
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code
• All code in src/services/ uses repositories implementations for database calls, filesystem operations, etc.
• All code in src/repositories/ is pretty basic/simple and does not have any immich specific logic (that belongs in src/services/)

Please describe to which degree, if any, an LLM was used in creating this pull request.

An LLM (Cascade) was used to identify the root cause and generate the one-line fix.

Screenshot of tested fix, before vs after

[immich-push-o-matic]

Label error. Requires exactly 1 of: changelog:.*. Found: 📱mobile. A maintainer will add the required label.

[Meliox]

Why would you pick a birthday in the future within the current year? The birthday relates to birth date which is always a past event.

[socksprox]

Why would you pick a birthday in the future within the current year? The birthday relates to birth date which is always a past event.

You select it from left to right. All month options should be available.