chore(mobile): smoother UI experience for iOS devices

[kao-byte]

Description

This animation issue affects iOS and iPadOS only, Android does not have this aniamtion
Please see attached videos at the Screenshots section to better understand this issue

Why is this change required?

1. When swiping back to Search/Albums/Library pages
2. The destination page (the one appearing from the left) doesn't move linearly (the speed is not constant)
3. It suddenly accelerates after a certain point and looks glithy

Changes
Changed both routes from CustomRoute to AutoRoute and removed the transitionsBuilder: TransitionsBuilders.fadeIn

How Has This Been Tested?

• Tested on iOS devices

Screenshots (if appropriate)

Original:
https://github.com/user-attachments/assets/b7f16fba-aa70-4800-a793-b470bcb81e75
After this fix:
https://github.com/user-attachments/assets/088851a9-ff9e-4804-8205-c0c19216602b

Checklist:

• 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.

Claude Code used to indentify the problem under human supervision and verification