Use custom splitter component

[mstange]

Production | Deploy preview

This replaces the splitter component with an implementation which lets us use max-height on the timeline, so that we don't have to compute the timeline height based on the visible tracks.

This PR also removes panelLayoutGeneration because it's no longer needed; it originally resolved #1205 and #736 but these days we have ResizeObservers in the relevant places.

┆Issue is synchronized with this Jira Task

[codecov]

Codecov Report

❌ Patch coverage is 46.93878% with 52 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.38%. Comparing base (c886717) to head (d644702).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/shared/ResizableWithSplitter.tsx	36.73%	31 Missing ⚠️
src/components/shared/Draggable.tsx	30.43%	16 Missing ⚠️
src/components/app/BottomBox.tsx	72.22%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4606      +/-   ##
==========================================
- Coverage   85.45%   85.38%   -0.07%     
==========================================
  Files         321      322       +1     
  Lines       32139    32152      +13     
  Branches     8850     8848       -2     
==========================================
- Hits        27463    27452      -11     
- Misses       4241     4265      +24     
  Partials      435      435              

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