Episodes can be kept on the device to listen offline (#412)

Offline mode (#389) kept lists, text and pictures, but the worker let
every /media/ request pass, so no episode played without a connection.

A downloaded episode's files pane has a keep button beside Play. It
fetches the copy that person plays, the file or, when DeadAir cuts
something for them, the version without its dead air (/media/{id}/deadair,
the save button's), whole into a cache of its own, ipx-episodes, and
asks once for persistent storage. A kept episode plays as
/media/...?offline, online or not, and the worker answers that from the
cache with the byte ranges an audio element asks for (206, 416, 200);
not kept there, it goes to the server as before. Pressing it again takes
it off; signing out clears it with everything else. Only someone's
choice is kept, so there is no budget: a full device is the quota's
refusal, said in words. The button is data-a="offline", not "keep",
which the Directory's show page already uses for subscribing.

Places saved from DeadAir's version were read in its clock only if ipx
had handed out that file's playlist since it started (state.playing). A
kept copy plays without one, so the player now names the file in the
position it saves (deadair: <enclosure>) whenever it plays DeadAir's
clock, and ipx loads that file's plan when it has none, for that item's
file only. Checked the beacon is queued by the worker offline: it is.

Tests: playing a kept episode offline, its place queued and sent once
back, and taking it off (the offline show plays quiet.mp3, the only real
MP3 among the fixtures); the worker's ranges; which copy is kept and
played, in page-unit; the server reading a flagged place in DeadAir's
clock without a playlist, and ignoring another item's file. The files
pane's one-row test counts five buttons and checks a phone's width too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqkrVDqdiQEMWAkNGQ2tYA

web/src/player.ts

@@ -5,7 +5,9 @@ const player: {guid: string|null, feed: string|null, entry: Entry|null, enc?: nu
 
 /// Where a file plays from: DeadAir's playlist when the server says this person plays it
 /// shortened (#171, #181), whose times the entry is already given in, else the file itself.
-const mediaSrc=(enc: {id: number, deadair?: number|null})=>enc.deadair!=null?`/media/${enc.id}/deadair.m3u8`:`/media/${enc.id}`;
+/// One kept on this device (#412) plays from there, online or not: the worker answers `?offline`.
+const mediaSrc=(enc: {id: number, deadair?: number|null})=>isKept(enc)?`${keepUrl(enc)}?offline`
+  :enc.deadair!=null?`/media/${enc.id}/deadair.m3u8`:`/media/${enc.id}`;
 
 /// hls.js, playing DeadAir's playlist where the browser cannot itself. Fetched the first time it
 /// is needed. Only Apple's browsers play it themselves: Chromium says it can ("maybe"), then fails
@@ -128,7 +130,10 @@ function savePos(){
   navigator.sendBeacon?.(
     // A beacon sends no headers, so the trace goes in the address (#192).
     `/api/entries/${encodeURIComponent(player.feed!)}/${encodeURIComponent(player.guid!)}/position?traceparent=${traceparent()}`,
-    new Blob([JSON.stringify({secs:Math.floor(audio.currentTime),duration})],{type:'application/json'}));
+    // DeadAir's clock says which file it is, for the server to read the place in it without having
+    // seen its playlist: a kept copy plays offline, and the server may have restarted since.
+    new Blob([JSON.stringify({secs:Math.floor(audio.currentTime),duration,
+      deadair:player.src?.includes('/deadair')?player.enc:undefined})],{type:'application/json'}));
 }
 audio.addEventListener('pause',savePos);
 audio.addEventListener('ended',()=>{savePos();markPlayed();$<HTMLButtonElement>('#pplay').innerHTML=ICON.play});