Upgrade react and react-dom to v19 and adapt gentype

[cknitt]

For React 19 typedefs, React.JSX.Element needs to be used in place of JSX.Element (which was already deprecated in React 18).

[chatgpt-codex-connector]

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Reviewed commit: 10798875c7

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.69%. Comparing base (27147f0) to head (79b3e53).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8501   +/-   ##
=======================================
  Coverage   74.69%   74.69%           
=======================================
  Files         457      457           
  Lines       62123    62123           
=======================================
  Hits        46400    46400           
  Misses      15723    15723           

Files with missing lines	Coverage Δ
compiler/gentype/emit_type.ml	91.48% <100.00%> (ø)

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