The submit button lives outside the .video-drop element, so as a Stimulus
target it never resolved and the submit lock silently did nothing. Move the
controller up to the verdict form, make the drop zone a target, and also
guard the form's submit event so Enter can't sneak a submit through
mid-upload.

Replacing the file while an upload is in flight now invalidates the old
upload's callbacks (token check) and removes the previous signed-id field
instead of stacking hidden inputs.

Also: drop the now-unused accepted class, give the progress bar a proper
progressbar role with min/max, and tidy the percent formatting.formatting.