Upgrade to React 19

[fatadel]

React 19 narrowed the callback-ref signature to (instance) => void | (() => void), where a returned non-void value is treated as a cleanup function. Several _takeXxxRef handlers were written as (el) => (this._el = el), which implicitly returns the assigned value and no longer satisfies the new type. They now use block bodies so they return undefined.

@types/react v19 also changed React.createRef() to return RefObject<T | null> instead of the legacy split between RefObject and MutableRefObject. MaybeFlameGraph's _flameGraph field declares the new shape directly so the assignment continues to typecheck.

React 19 appends "()" to component-stack frames whose component lacks a source location, so the ErrorBoundary regex now checks that suffix on the NonLocalizedErrorBoundary frame. React 19 also follows the HTML spec for boolean attributes on custom elements (prop={false} drops the attribute entirely); the Home tests mock ListOfPublishedProfiles as a custom-element string, so the affected snapshots no longer carry withactionbuttons="false".

Closes #5590

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.78%. Comparing base (667825c) to head (3cd40d3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6067   +/-   ##
=======================================
  Coverage   83.78%   83.78%           
=======================================
  Files         329      329           
  Lines       34570    34572    +2     
  Branches     9590     9590           
=======================================
+ Hits        28965    28967    +2     
  Misses       5176     5176           
  Partials      429      429           

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[canova]

Did some manual testing and it looks like everything is working well. Thanks!