video-extend-edit: carry an existing clip forward from one of its own frames

The user has footage. They want it longer, or they want a different ending. The mechanism is always the same and it is always a frame: pull one out of their clip locally for nothing, hand it to a new video job as its first frame, and join the result onto the original.

their clip ──> last-frame  ──┐
           └─> frame-at N ───┴─> that PNG as --frame-first-image ──> new segment ──> join

Both extractions are local: no API call, no key, no cost. The only billable step is the new segment.