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>

web/index.html

@@ -66,6 +66,7 @@
     </div>
   </div>
   <div id="pright">
+    <button class="iconbtn" id="pdead" aria-pressed="false" aria-label="DeadAir" hidden></button>
     <select id="rate" title="Speed">
       <option value="0.8">0.8×</option><option value="1" selected>1×</option>
       <option value="1.25">1.25×</option><option value="1.5">1.5×</option>