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>

lib/services/sync_service.dart

@@ -105,12 +105,17 @@ class SyncService {
               )
             : _SyncConfig.fromPrefs(prefs, store.accountPrefKey, account.id);
         final password = await store.readPassword(account.id);
-        if (password == null ||
-            config.paths.isEmpty ||
-            config.intervalMinutes == null) {
+        if (password == null || config.paths.isEmpty) {
           await cancelAccount(account.id);
           continue;
         }
+        if (config.intervalMinutes == null) {
+          // Background sync is off, but this account still has paths to sync
+          // by hand: keep its credentials (native uses them for "Sync now"
+          // and conflict notification actions).
+          await cancelAccount(account.id, forget: false);
+          continue;
+        }
         await invokeIfAvailable(_channel, 'reschedule', {
           'accountId': account.id,
           'serverUrl': account.serverUrl,
@@ -132,8 +137,17 @@ class SyncService {
 
   /// Stops [accountId]'s periodic job (the account was removed, or has
   /// nothing left to sync).
-  static Future<void> cancelAccount(String accountId) async {
-    await invokeIfAvailable(_channel, 'cancel', {'accountId': accountId});
+  ///
+  /// [forget] (the default) is for an account that was signed out or removed:
+  /// native also drops its stored credentials. Pass false when only the
+  /// background job is being turned off - the account's paths are still
+  /// synced by hand ("Sync now"), and a conflict notification's actions
+  /// still need its credentials.
+  static Future<void> cancelAccount(String accountId, {bool forget = true}) async {
+    await invokeIfAvailable(_channel, 'cancel', {
+      'accountId': accountId,
+      'forget': forget,
+    });
   }
 
   /// Runs a one-off sync pass immediately, independent of the periodic