iOS: device sync (Swift port of the Android engine) behind the sync_service channel

PROPFIND walk / diff / GET / PUT mirror with conflicts, per-account state,
BGTaskScheduler background runs, and conflict notifications with Keep local /
Use server. Reviewed by Codex against the Kotlin original; its data-loss
findings are fixed and each has a regression test (fake-server end-to-end
tests, 68 Swift tests total): truncated/unusable PROPFIND answers are never a
manifest, unreadable state aborts the run, server paths can't leave the mirror,
case/Unicode collisions are skipped, atomic downloads, a locally edited file
deleted on the server is kept, '/' scope matching, serialised removal,
tracked/cancellable background runs, per-account conflicts, and sign-out vs
background-off credentials (cancel gains a 'forget' flag).

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

ios/Runner/Info.plist

@@ -59,6 +59,16 @@
 	<true/>
 	<key>NooKeychainAccessGroup</key>
 	<string>$(AppIdentifierPrefix)dev.ayushya.noo.shared</string>
+	<key>BGTaskSchedulerPermittedIdentifiers</key>
+	<array>
+		<string>dev.ayushya.noo.sync.refresh</string>
+		<string>dev.ayushya.noo.sync.processing</string>
+	</array>
+	<key>UIBackgroundModes</key>
+	<array>
+		<string>fetch</string>
+		<string>processing</string>
+	</array>
 	<key>NSFaceIDUsageDescription</key>
 	<string>Noo uses Face ID to unlock the app and to protect hidden files.</string>
 	<key>NSPhotoLibraryUsageDescription</key>