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/project.yml

@@ -25,7 +25,7 @@ targets:
       properties:
         CFBundleDisplayName: iPX
         CFBundleShortVersionString: $(MARKETING_VERSION)
-        CFBundleVersion: "5"
+        CFBundleVersion: "6"
         # Playing on with the screen locked. A plist key, not a signed entitlement, so it works on
         # a free personal team; CarPlay's own dashboard app is an entitlement Apple has to grant.
         UIBackgroundModes: [audio, fetch]