The video preview is object-fit: contain (the browser default for <video>), so a
non-16:9 source is letter/pillarboxed inside the element box. The crop marquee was
drawn in fractions of the whole box (bars included) and those fractions were then
multiplied by the source resolution, so the emitted ffmpeg crop diverged from what
the selector showed. Anchor the overlay to the computed content rect (and set
object-contain explicitly) so a marquee fraction is a fraction of the real picture;
extract the display-px<->source-px math into cropGeometry.ts with unit tests, and
clamp the even-rounded window in-bounds so a full-frame crop can't overflow.
