iOS: episodes kept on the phone play with no connection (#425)

A WKWebView runs no service worker, so the page's Keep button (#412)
was not shown in the app and everything played from the server. Now the
page asks the app to keep the file (keep, unkeep, clearKept), and the
app answers with what it holds (kept, keepFailed) whether or not
anything plays. The app fetches with a background URLSession, so a
download goes on with the app closed, into Application Support (not
backed up; the files can be fetched again), and Playback plays from the
file when there is one. The DeadAir copy is kept under its own path, as
the browser keeps it. Places and finishes reached offline wait in an
outbox, newest per address, and go when the network is back.

Checked in the simulator against a local server: kept /media/42 and
/media/42/deadair, then with the server refusing /media, a fresh launch
played each from the phone and asked the server for nothing. The page
itself still needs a connection to open (#435).

Deploy the app (build 6) before the page: a new page in an old app
shows a Keep button nothing answers.

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

ios/CHANGELOG.md

@@ -9,3 +9,4 @@ The iOS app, `ios/`. Format: [Keep a Changelog](https://keepachangelog.com/en/1.
 - iPX on the iPhone: the page in an app, playing through the phone's own player, so an episode plays on with the screen locked and the lock screen, Control Center and a car's buttons drive it (#418).
 - Player token (shake, then Player Token): an ipx API token, kept in the keychain, which the player, the badge and CarPlay send to `https://ipxapi.sdf1.net` instead of using the page's sign-in cookie, so playback does not stop when that cookie expires (#419).
 - The icon shows All Subscriptions' unread count, as the page's badge does: set when the app comes forward, after an item is marked read, and by a refresh iOS runs now and then while the app is closed, so it can be hours behind (#421).
+- Episodes kept on the phone, with the page's Keep button, play with no connection: the app fetches the file in the background, even with the app closed, and plays it from the phone; where you got to and what you finished offline is sent once the connection is back. Signing out removes them (#425).