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/CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Keep an episode on this device to listen offline: the button beside Play on a downloaded episode keeps the file, without its dead air if DeadAir shortens it for you, and it then plays from the device whether you are online or not, with your place sent once the connection is back. Press it again to take it off; signing out takes everything off (#412).
 - iPX opens and reads without a connection: the lists it has fetched (All Subscriptions, Unread and the feeds it preloads or you opened) are kept on the device with the pictures in their unread items, up to 200 MB of them, and items read or flagged and places in an episode while offline are sent when the connection is back. Signing out clears it all (#389).
 - An open page, or the home-screen app, takes up a new version of iPX by itself once it reconnects after a deploy, back on the item you had open; while something is playing, a dialog is open or a search is typed, it offers a reload instead (#388).