Add threshold

[ahuang11]

While using lumen-anndata, I was a bit annoyed that the default was like 50/50, and when I clicked <, it snapped to 40/60. I think it's better to allow users to customize within a threshold, e.g. within 5% of the snap size, it collapses the entire thing

Before:

before.mp4

After:

afte.mp4

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.25%. Comparing base (77e6832) to head (c895323).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   87.12%   87.25%   +0.12%     
==========================================
  Files           3        3              
  Lines         101      102       +1     
==========================================
+ Hits           88       89       +1     
  Misses         13       13              

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