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/util.ts

@@ -41,6 +41,8 @@ const ICON: Record<string,string>={
   pinOn:fa('-18 -18 420 548','<path fill="currentColor" d="M32 32C32 14.3 46.3 0 64 0L320 0c17.7 0 32 14.3 32 32s-14.3 32-32 32l-29.5 0 11.4 148.2c36.7 19.9 65.7 53.2 79.5 94.7l1 3c3.3 9.8 1.6 20.5-4.4 28.8s-15.7 13.3-26 13.3L32 352c-10.3 0-19.9-4.9-26-13.3s-7.7-19.1-4.4-28.8l1-3c13.8-41.5 42.8-74.8 79.5-94.7L93.5 64 64 64C46.3 64 32 49.7 32 32zM160 384l64 0 0 96c0 17.7-14.3 32-32 32s-32-14.3-32-32l0-96z"/>'), // solid/thumbtack
   scan:fa('0 0 512 512','<path fill="currentColor" d="M65.9 228.5c13.3-93 93.4-164.5 190.1-164.5 53 0 101 21.5 135.8 56.2 .2 .2 .4 .4 .6 .6l7.6 7.2-47.9 0c-17.7 0-32 14.3-32 32s14.3 32 32 32l128 0c17.7 0 32-14.3 32-32l0-128c0-17.7-14.3-32-32-32s-32 14.3-32 32l0 53.4-11.3-10.7C390.5 28.6 326.5 0 256 0 127 0 20.3 95.4 2.6 219.5 .1 237 12.2 253.2 29.7 255.7s33.7-9.7 36.2-27.1zm443.5 64c2.5-17.5-9.7-33.7-27.1-36.2s-33.7 9.7-36.2 27.1c-13.3 93-93.4 164.5-190.1 164.5-53 0-101-21.5-135.8-56.2-.2-.2-.4-.4-.6-.6l-7.6-7.2 47.9 0c17.7 0 32-14.3 32-32s-14.3-32-32-32L32 320c-8.5 0-16.7 3.4-22.7 9.5S-.1 343.7 0 352.3l1 127c.1 17.7 14.6 31.9 32.3 31.7S65.2 496.4 65 478.7l-.4-51.5 10.7 10.1c46.3 46.1 110.2 74.7 180.7 74.7 129 0 235.7-95.4 253.4-219.5z"/>'), // solid/arrows-rotate
   download:fa('0 0 448 512','<path fill="currentColor" d="M256 32c0-17.7-14.3-32-32-32s-32 14.3-32 32l0 210.7-41.4-41.4c-12.5-12.5-32.8-12.5-45.3 0s-12.5 32.8 0 45.3l96 96c12.5 12.5 32.8 12.5 45.3 0l96-96c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0L256 242.7 256 32zM64 320c-35.3 0-64 28.7-64 64l0 32c0 35.3 28.7 64 64 64l320 0c35.3 0 64-28.7 64-64l0-32c0-35.3-28.7-64-64-64l-46.9 0-56.6 56.6c-31.2 31.2-81.9 31.2-113.1 0L110.9 320 64 320zm304 56a24 24 0 1 1 0 48 24 24 0 1 1 0-48z"/>'), // solid/download
+  keep:fa('0 0 512 512','<path fill="currentColor" d="M256 464a208 208 0 1 1 0-416 208 208 0 1 1 0 416zM256 0a256 256 0 1 0 0 512 256 256 0 1 0 0-512zM244.7 387.3c6.2 6.2 16.4 6.2 22.6 0l104-104c4.6-4.6 5.9-11.5 3.5-17.4S366.5 256 360 256l-72 0 0-104c0-13.3-10.7-24-24-24l-16 0c-13.3 0-24 10.7-24 24l0 104-72 0c-6.5 0-12.3 3.9-14.8 9.9s-1.1 12.9 3.5 17.4l104 104z"/>'), // regular/circle-down
+  kept:fa('0 0 512 512','<path fill="currentColor" d="M256 0a256 256 0 1 0 0 512 256 256 0 1 0 0-512zM244.7 387.3l-104-104c-4.6-4.6-5.9-11.5-3.5-17.4s8.3-9.9 14.8-9.9l56 0 0-96c0-17.7 14.3-32 32-32l32 0c17.7 0 32 14.3 32 32l0 96 56 0c6.5 0 12.3 3.9 14.8 9.9s1.1 12.9-3.5 17.4l-104 104c-6.2 6.2-16.4 6.2-22.6 0z"/>'), // solid/circle-down
   save:fa('0 0 448 512','<path fill="currentColor" d="M64 32C28.7 32 0 60.7 0 96L0 416c0 35.3 28.7 64 64 64l320 0c35.3 0 64-28.7 64-64l0-242.7c0-17-6.7-33.3-18.7-45.3L352 50.7C340 38.7 323.7 32 306.7 32L64 32zm32 96c0-17.7 14.3-32 32-32l160 0c17.7 0 32 14.3 32 32l0 64c0 17.7-14.3 32-32 32l-160 0c-17.7 0-32-14.3-32-32l0-64zM224 288a64 64 0 1 1 0 128 64 64 0 1 1 0-128z"/>'), // solid/floppy-disk
   trash:fa('0 0 448 512','<path fill="currentColor" d="M136.7 5.9C141.1-7.2 153.3-16 167.1-16l113.9 0c13.8 0 26 8.8 30.4 21.9L320 32 416 32c17.7 0 32 14.3 32 32s-14.3 32-32 32L32 96C14.3 96 0 81.7 0 64S14.3 32 32 32l96 0 8.7-26.1zM32 144l384 0 0 304c0 35.3-28.7 64-64 64L96 512c-35.3 0-64-28.7-64-64l0-304zm88 64c-13.3 0-24 10.7-24 24l0 192c0 13.3 10.7 24 24 24s24-10.7 24-24l0-192c0-13.3-10.7-24-24-24zm104 0c-13.3 0-24 10.7-24 24l0 192c0 13.3 10.7 24 24 24s24-10.7 24-24l0-192c0-13.3-10.7-24-24-24zm104 0c-13.3 0-24 10.7-24 24l0 192c0 13.3 10.7 24 24 24s24-10.7 24-24l0-192c0-13.3-10.7-24-24-24z"/>'), // solid/trash-can
   open:fa('0 0 512 512','<path fill="currentColor" d="M320 0c-17.7 0-32 14.3-32 32s14.3 32 32 32l82.7 0-201.4 201.4c-12.5 12.5-12.5 32.8 0 45.3s32.8 12.5 45.3 0L448 109.3 448 192c0 17.7 14.3 32 32 32s32-14.3 32-32l0-160c0-17.7-14.3-32-32-32L320 0zM80 96C35.8 96 0 131.8 0 176L0 432c0 44.2 35.8 80 80 80l256 0c44.2 0 80-35.8 80-80l0-80c0-17.7-14.3-32-32-32s-32 14.3-32 32l0 80c0 8.8-7.2 16-16 16L80 448c-8.8 0-16-7.2-16-16l0-256c0-8.8 7.2-16 16-16l80 0c17.7 0 32-14.3 32-32s-14.3-32-32-32L80 96z"/>'), // solid/arrow-up-right-from-square