Walkthroughs: stop autoplaying muted, and show the length

Marty asked why they start muted. Browsers only permit autoplay when a video
is muted, so "expand and play" necessarily meant "expand and play silently" —
and for these that is the wrong trade, because the narration IS the training.
A member's first visit was showing them a silent walkthrough.

So the panel now opens paused with the play button showing. Pressing it is a
user gesture, which means it plays with sound, every time. preload stays
'none' until the panel opens, then goes to 'metadata' so an open panel shows
a real first frame instead of a black rectangle.

Each bar now also carries its runtime ("· 71 sec"), so nobody has to guess
whether they are committing to one minute or ten.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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@@ -46,6 +46,6 @@
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