The <video> element used `max-w-full max-h-full`, which only ever shrinks
oversized media. A source smaller than the window (480p on a 1080p display)
rendered at its intrinsic size — a small picture floating in a black frame.
Fill the container and let `object-contain` do the scaling, so the picture
fits whichever axis constrains it in both directions while preserving aspect
ratio. The sizing rules move to `videoFit.ts` so they are unit-testable
outside the component.
