User request: puzzle timer with sound alert

See https://lichess.org/forum/lichess-feedback/feature-request-puzzle-clock-with-alert-sound

tldr: Show a clock during the normal puzzle feature that counts up from zero. Allow users to optionally set a time at which an alert will play (e.g. after 10 seconds).

Previously, other users have asked for a time limit during the normal puzzle solving. (No, Puzzle Storm is not the same thing. We want a fixed limit for each individual puzzle.) It seems this is an unpopular idea with the admins/developers, and they probably have good reasons. So I suggest an alternative: show a clock to the side of the screen during normal puzzle solving that counts up. Optionally, let the user choose an amount of time after which an alert will sound. The alert could be the same sound as used during a game when time is short.

Start with the linked lichess.org forum request and locate the normal puzzle feature entry point in the extension. The requested result is a visible clock counting up from zero, an optional user-set alert time, and an alert sound when that time is reached; no file or test is named in the issue.