Support grids portaled into another window

[nstepien]

Fixes #4184

When the grid is portaled into another window (e.g. an iframe, or a popup via window.open()), its DOM nodes belong to that window, while its JS still runs in the opener's window. A few places assumed both were the same:

• useGridDimensions: the module-level ResizeObserver was created in the opener's realm, so resize notifications were processed on the opener's rendering timeline, which can be throttled while the user interacts with the popup. We now lazily create one ResizeObserver per window (keyed in a WeakMap), derived from the grid element's ownerDocument.defaultView.
• Keyboard navigation: target instanceof Element is always false for elements from another window, so the grid ignored arrow keys, Tab, Home/End, PageUp/PageDown, and typing to start editing. Replaced with a nodeType check (isElement).
• Tab out of an editor: onEditorNavigation used instanceof HTMLInputElement/HTMLTextAreaElement/HTMLSelectElement. Replaced with isElement(target) && target.matches('input, textarea, select').
• Commit on outside click: EditCell listened for mousedown on the opener's window, which never receives clicks from the other window. It now listens on the editor's window, and schedules/cancels the fallback check with that window's scheduler.postTask/requestAnimationFrame.

Tests live in test/browser/crossWindow.test.tsx (renamed from gridDimensions.test.tsx), and render the grid into an iframe via createPortal, using page.frameLocator() for real user interactions. The commit history has each test failing first.

The requestAnimationFrame part isn't covered by a test: it only matters when postTask is unavailable and the opener window is hidden, which can't be simulated here.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.41%. Comparing base (5110b86) to head (404a35b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4194      +/-   ##
==========================================
+ Coverage   98.30%   98.41%   +0.10%     
==========================================
  Files          45       45              
  Lines        1943     1950       +7     
  Branches      732      727       -5     
==========================================
+ Hits         1910     1919       +9     
+ Misses         33       31       -2     

Files with missing lines	Coverage Δ
src/DataGrid.tsx	98.88% <100.00%> (ø)
src/EditCell.tsx	97.01% <100.00%> (ø)
src/hooks/useGridDimensions.ts	100.00% <100.00%> (ø)
src/utils/domUtils.ts	100.00% <100.00%> (ø)
src/utils/keyboardUtils.ts	100.00% <100.00%> (+10.00%)	⬆️

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