ASAP-899 Fix scroll issue on assign users modal

[gabiayako]

This PR addresses two issues occurring in both Dev and Prod environments, as demonstrated in the video below:

Screen.Recording.2025-02-10.at.09.43.29.mov

1.	The background scrolls down each time the assigned users’ suggestions list is clicked.
2.	The background remains scrollable when the modal is open.

Issue 1: Background scrolling when interacting with the suggestions list

The root cause appears to be the height of the assigned users’ list, which overflows the visible screen space and causes the background to shift. To mitigate this, the following changes were implemented:

• The assigned users’ list was given a fixed height.
• When the list is clicked, the modal automatically scrolls to the bottom, minimizing any unexpected shifts.

Issue 2: Background remains scrollable while the modal is open

To resolve this, I implemented a solution based on this Stack Overflow answer. This ensures the background becomes unscrollable when the modal is active, improving the overall user experience.

---

After the fix:

Screen.Recording.2025-02-10.at.09.51.37.mov

Although it's not perfect because there still a shift in the first click, it doesn't scroll each time the user clicks on the assigned users’ suggestions list.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.50%. Comparing base (098ea6f) to head (77e4e8b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4520   +/-   ##
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  Coverage   99.49%   99.50%           
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  Files        1092     1092           
  Lines       22191    22206   +15     
  Branches     7031     7034    +3     
=======================================
+ Hits        22080    22095   +15     
  Misses        111      111           

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