Fix read receipt animation

[dbkr]

The way it was done involved remembering dom nodes and then getting their position later when animating the receipt to its next position, but I'm not sure how this worked since the DOM node may not neccessarily be in the DOM anymore. Instead, just remember the bounding box coordinates. At worst it might go weird if the window is resized but seems fine in practice. Also, keeping references to dom nodes feels like a fast road to memory leaks.

Fixes https://github.com/element-hq/element-web/issues/27916

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [ ] Linter and other CI checks pass.
• [ ] Sign-off given on the changes (see CONTRIBUTING.md).