fix: z-index of top bar on show/hide people view

[stewx]

Description

This makes the top bar appear on top while scrolling through the show/hide people list.

Fixes #21474

How Has This Been Tested?

Screenshots (if appropriate)

I have blurred the photos and names here for privacy.

Before:

topbarbefore.mp4

After:

topbarfix.mp4

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.

...

[danieldietzler]

This is probably fine but might have unintended side effects on other pages; haven't tested the PR. IIRC this should be fine here though