[Feature Request/Offer] Add ability to adjust preview frequency and request previews at-will in the UI

Currently I have this implemented as a slider that lets you select the number of seconds of generated video between performing a VAE Decode and writing out a progress preview mp4, settable between generation launches. I'm thinking of also having a live button to drop a preview after the second currently rendering finishes, so regardless of whether periodic previews are being generated the user can opt to see the work in progress so far.

VAE Decode is an insanely slow procedure compared to sampling and having an at-will preview option sure is useful to me anyway. Feelings anybody? I'm working on some other stuff in the same fork but wanted to get this idea out there. I think it's different enough from the current UI features in the pull requests.

SVC

Start by reviewing the existing UI preview controls and the VAE Decode path that writes progress preview MP4 files. Trace how preview frequency is selected between generation launches and how the currently rendering second is tracked. Done means users can adjust the preview interval and request an immediate preview from the UI without disrupting generation.

Written by the indexing model from the issue text.