[1/2] Continue a clip with a generated video

[dazzatronus]

Building a sequence of shots meant exporting a clip's last frame and uploading it as the next clip's start image. edit.continueFromClip(clipId) and a Continue button on the clip, asset and generate toolbars now add a pending AI video straight after a video or image clip, with options.startSrc set to that clip's src. Nothing is spent until the user generates.

• The link is the file URL, not an alias, so a render of the edit can reuse the generated file.
• The model is the first catalogue model that accepts startSrc. With a video source the host's generator receives a video URL, so it must accept one.
• The button refreshes on edit:changed, so it appears as soon as a generation lands on the selected clip.

Verify: npm run verify:ci (89 suites, 2172 tests).

	PR	Adds	Why separate
▶ 1/2	this PR	Continue action and button	Useful on its own
2/2	#217	Repointing continuations on regenerate	Changes regeneration and undo; revertible alone