demo-stb: 720p

I had made it 480p to reduce CPU & bandwidth costs, but we want
test-result screenshots to be at 720p; we'll downscale for the
live video stream.
demo-stb: Arrow keys move the selection

Because the blue selection rectangle is *between* the carousel item's
black background and its contents, and because css z-index is only
relative between *sibling* elements, I had to pull out the carousel
items' backgrounds into a separate element:

    body
    + carousel
    | + item 1
    | + ...
    + carousel-backgrounds
    | + item 1's background
    | + ...
    + carousel-selection

The "carousel-backgrounds" children are created automatically based on
the "carousel" children.
demo-stb: Pressing Return plays the selected video
demo-stb: Remove hint for unimplemented "search" feature

I'm cutting this feature because it'll take too much time.
demo-stb: Don't trap "Control-R" and similar keys

For my convenience while developing this app.

The arrow keys & backspace are still trapped, so they don't move the
viewport or navigate back in history.
fixup! extra/demo-stb: Press P for pause/play