Web: the fullscreen button placement is slightly off

The button to toggle fullscreen in the web browser when running in windowed mode is slightly off. We could pass the rectangle of the desktop fullscreen button that we have in Godot for the Win, Mac, and Linux version to the browser to calculate the relative placement that this button should take. or we could directly calculate the relative placement of the desktop button in the app and just pass the calculated information to the browser as well, whichever works.

No files, tests, or entry points are named. Start by locating the browser fullscreen button and the desktop fullscreen button in the Godot project, then trace how their positions are calculated or passed to the browser. Done means the web button is relatively aligned with the desktop button in windowed mode across Windows, macOS, and Linux.

Written by the indexing model from the issue text.