Client-side frame extraction is far too slow to be usable for large volumes of data.

You want to precompute the contact sheets and serve them to users. You can encode them with VP9, mux to IVF format, and use the WebCodec API to decode them in the browser (2000B-3000B per 240x135 frame, so ~3MB/hour for a thumbnail every 4 seconds). Alternatively, you can make the contact sheets with JPEG, but there are dimension restrictions, reflow is slightly fiddly, and it doesn't exploit intra-frame compression.

I made a simple Python/Flask utility for lossless cutting that uses this to present a giant contact sheet to quickly select portions of a video to extract.