Podcast player: improve on mobile

Podcast block gives this on mobile width:

It's pretty but hard to use on mobile, unfortunately. :-(

- Hit-areas are so small that it's not meaningful. Buttons to speed track +/-30sec or +/-10sec is much more useful on mobile, as well long enough timeline to tap on.
- Volume isn't really needed on mobile. Folks just press device buttons.
- Play/pause is too small for tapping — works, but the hit area could be bigger than the icon is since there is space.
- Contrast difference in the timeline in outdoors on a small screen isn't enough. I'd argue it isn't enough on the desktop either.

Hiding volume (thus making space for timeline) and making play button slightly bigger would be already big improvements.

For comparison, here are a couple of competitors:

Squarespace

Apple

@lessbloat

No file or test is named in the issue; start by locating the Podcast block and its player entry point, then inspect the mobile layout and controls. Done means the mobile player has larger tap targets, a larger play button, no unnecessary volume control, more timeline space, and stronger timeline contrast.

Written by the indexing model from the issue text.