DeadAir in the page: a switch per show, one beside the speed, and the time saved (#172)

A DeadAir switch in each show's settings, and the same switch beside
the speed for the show that is playing, drawn as a waveform with its gap
closed up, outlined when off and solid when on, with aria-pressed. A
file the server gives a copy for plays from /media/{id}/deadair. An
episode's line says how much shorter it is, and Settings says how much
DeadAir has saved you.

Turning it on or off mid-episode saves the place with a request it waits
for, not a beacon, then asks for the entry again from the new
GET /api/entries/{feed}/{guid}, whose position is in the clock of the
version it now plays, and carries on there. A copy not made yet plays
from the next time. Turning a show on wakes the copy maker, so the copy
is there in seconds rather than at its next pass.

The browser test plays a real MP3 with long silences, quiet.mp3, from a
show it adds and removes itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- DeadAir, on the server: in each feed you turn it on for, episodes get a copy with their long silences shortened, so podcasts finish sooner without sounding sped up, and your place carries over between the copy and the original. It is off for every feed until you do; the setting in the page comes next (#171).
+- DeadAir: turn it on for a show in its settings, or with the button beside the speed while it plays, and its episodes play with long silences shortened, so podcasts finish sooner without sounding sped up. Your place carries over when you turn it on or off, each episode says how much shorter it is, and Settings shows the time it has saved you. It is off for every show until you turn it on (#171, #172).
 
 ### Fixed