Show preview of frame when hovering over seek bar

Feature request to support showing a preview image of a point in the video when the seek bar is hovered over.

One way to do it is in the style of YouTube:


Or, since ClassTranscribe mostly hosts screen recordings of large amounts of text, instead of showing a small preview over the cursor, the preview image could take up most of the screen, to let users see finer details:

Before hover:

While hovering:

Some way to visually navigate the video without having to do a binary search through clicking and waiting for the video to buffer would be super helpful for skimming videos.

Start by locating the video player and seek-bar entry points in the React + Redux frontend, then review how seeking and hover interactions currently work. Define which preview presentation to implement from the alternatives in the issue, and consider the work done when users can visually inspect the video at hovered seek-bar positions without repeated buffering waits.

Written by the indexing model from the issue text.