fix(kvm): Map mouse to framebuffer via on-screen box, honour object fit

[michaelmadell]

The pointer position sent to AMT was wrong whenever the canvas wasn't shown 1:1 at its layout position:

• Fullscreen: browsers give a full-screen element object-fit: contain, so the picture is letterboxed inside a screen-sized element. The mapping ignored the bars, putting the remote cursor up and to the right (e.g. aiming at 700,500 of a 1024x768 desktop on a 1600x900 screen send 870,388).
• X was scaled by the height ratio and Y by the width ratio, so any non-uniform stretch skewed both axes.
• The cached largest bounding-rect top/left was added to every event, so scrolling the page shifted clicks by the scroll distance.

Map clientX/clientY through getBoundingClientRect instead (handles scroll, CSS scaling and transforms), scale each axis by its own ratio, and account for object-fit contain/scale-down letterboxing. Coordinates are clamped to the framebuffer. resetOffsets() is kept as a no-op for API compatibility

PR Checklist

• Unit Tests have been added for new changes
• API tests have been updated if applicable
• All commented code has been removed
• If you've added a dependency, you've ensured license is compatible with Apache 2.0 and clearly outlined the added dependency.

What are you changing?

making the pointer position align when the canvas is full-screened or scaled to avoid the incorrect pointer position on the remote machine