Make withSize use a wrapper element so that it can stop calling findDOMNode

[mstange]

Summary

findDOMNode is deprecated in React 18 and removed in React 19 strict mode. The withSize HOC was the last in-tree caller.

This PR makes withSize render a <div style="display: contents"> wrapper around the wrapped component and observe wrapper.firstElementChild via the existing ResizeObserver wrapper, removing the need for findDOMNode. The HOC is also converted to a function component using useState + useRef + useEffect.

The docstring previously claimed the size only updates on window resize; that hasn't been true since the HOC moved to ResizeObserver, so it's been corrected to "whenever the element's size changes".

Test fixes

EmptyThreadIndicator.test.tsx and Timeline.test.tsx were mocking findDOMNode to inject sizes into the wrapped component. With findDOMNode gone those mocks silently became no-ops — EmptyThreadIndicator snapshots showed zero-width indicators and Timeline's per-describe override fell back to the file-level autoMockElementSize. The second commit replaces the mocks with autoMockElementSize, restoring the previous size assertions.

Snapshot deltas

Every snapshot covering a withSize-wrapped component now includes the new <div style="display: contents"> wrapper. The only width/height changes are in EmptyThreadIndicator.test.tsx.snap (still 30/10/20px after the mock fix).

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.76%. Comparing base (b868d0e) to head (d862f92).
⚠️ Report is 58 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/shared/WithSize.tsx	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5988      +/-   ##
==========================================
- Coverage   85.34%   83.76%   -1.59%     
==========================================
  Files         318      329      +11     
  Lines       31922    34415    +2493     
  Branches     8834     9522     +688     
==========================================
+ Hits        27244    28827    +1583     
- Misses       4246     5159     +913     
+ Partials      432      429       -3     

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