fix: Avoid stale state in singleton-handler

[just-boris]

Issue #, if available:

Description of changes:

There are only two hard things in Computer Science: cache invalidation and naming things.

In this PR we got incorrect caching.

This affects our useMobile hook. Example:

test('first', () => {
  globalThis[forceMobileModeSymbol] = false;
  render(<AppLayout />); // I see desktop view

  // resize to mobile
  act(() => {
    globalThis[forceMobileModeSymbol] = true;
    window.dispatchEvent(new CustomEvent('resize'));
  });

  // clean up flag to its original state
  globalThis[forceMobileModeSymbol] = undefined;
});

test('second', () => {
  // no overrides in this test, it should be desktop by default, but the mobile size is leaked from the previous test
  render(<AppLayout />);
});

With the fix the "second" test will work as expected

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.39%. Comparing base (dca1353) to head (4d52cc5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
- Coverage   99.39%   99.39%   -0.01%     
==========================================
  Files          33       33              
  Lines         833      830       -3     
  Branches      234      224      -10     
==========================================
- Hits          828      825       -3     
  Misses          5        5              

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[teodoranemes]

Liked the CS quote from the Description