fix(video): prevent space bar from pausing video in fullscreen

[agh]

Fixes #265

Problem

Pressing Space in fullscreen mode pauses the video stream. Reported across Chrome, Edge, and Safari. The stream resumes on the next Space press but this makes fullscreen unusable for typing.

Root cause

Browsers treat <video> elements specially — Space toggles play/pause as a built-in behavior at the element level. The existing keyDownHandler on document calls preventDefault(), but DOM event propagation runs element → parent → ... → document. The video element processes the Space keydown and pauses playback before the document-level handler ever fires.

The existing workaround (videoKeyUpHandler at line 390) attaches a keyup listener on the video element that detects the pause and calls .play() to undo it, but:

1. There is a visible pause/unpause flicker on every Space press
2. It does not work in Safari (noted in the original code comment)

Fix

Add a keydown listener directly on the <video> element that calls preventDefault() for Space. This intercepts the event at the element level before the browser's native toggle runs, so the pause never occurs.

The existing keyup force-play handler is retained as a safety net for any browser that still manages to pause the stream.