Move paddings inside the tree header cells.

[mstange]

When we make columns sortable, this will offer a bigger click target and mousedown feedback area.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.82%. Comparing base (61e5d10) to head (1b6eaf9).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6002   +/-   ##
=======================================
  Coverage   83.81%   83.82%           
=======================================
  Files         328      328           
  Lines       34255    34254    -1     
  Branches     9572     9572           
=======================================
  Hits        28712    28712           
+ Misses       5115     5114    -1     
  Partials      428      428           

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