fix(web): timeline date group width

[ben-basten]

Description

Fix the calculation for the date group width, so there's never a scenario where photos will be hidden. Only use photos in the first row of the day group to determine its visible width.

On mobile devices, photos in the second row could have a top position of <100px, which was throwing off the calculation of the date group width.

• Fixes Photos missing in timeline on mobile devices #19897
• Fixes Web: timeline doesn't show random assets in date groups at specific browser widths #17632

How Has This Been Tested?

• Navigate to the shared link in the preview env, using either a mobile phone or the Chrome devtools with the device simulator set to "iPhone SE": https://pr-19964.preview.internal.immich.cloud/share/XBbGEcsN_3XO6BWTaDY6EpVnoM7IFP8N9gRFceFj0skNYuHJmRe1zVpTS_UIkj2KU8I
• Check that the main timeline is still showing day groups as expected

Annotated screenshot

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/)

[github-actions]

Deploying preview environment to https://pr-19964.preview.internal.immich.cloud/