Close the two ways a merge outlived its login

The merge dialog waits on the user, so the login can go between choosing
a playlist and confirming it. Reading it then returned quietly: the row
stayed in its importing state with nothing running, and a batch went on
to the next playlist. It now reports what happened and clears up after
itself, like every other way an import stops.

The merge that dialog starts is detached from it, so dismissing the
dialog was never the merge finishing — a batch resumed alongside it and
could begin the next playlist under whoever replaced the login. It is
waited for now, which is what makes the batch sequential.

Yattee/Views/Settings/ImportPlaylistsView.swift

@@ -26,6 +26,9 @@ struct ImportPlaylistsView: View {
     /// different account or leave an empty playlist behind for one that is
     /// gone.
     @State private var loginChangedDuringImport = false
+    /// The merge started from the confirmation dialog, which runs detached
+    /// from the batch that queued it. Held so the batch can wait for it.
+    @State private var mergeImportTask: Task<Void, Never>?
 
     // Merge warning state
     @State private var showMergeWarning = false
@@ -92,7 +95,7 @@ struct ImportPlaylistsView: View {
             ) {
                 Button(String(localized: "import.playlists.mergeWarning.merge")) {
                     if let playlist = pendingMergePlaylist, let localPlaylist = existingLocalPlaylist {
-                        Task { await performImport(playlist, into: localPlaylist) }
+                        mergeImportTask = Task { await performImport(playlist, into: localPlaylist) }
                     }
                 }
                 Button(String(localized: "common.cancel"), role: .cancel) {
@@ -338,8 +341,21 @@ struct ImportPlaylistsView: View {
     }
 
     private func performImport(_ playlist: Playlist, into localPlaylist: LocalPlaylist) async {
-        guard let appEnvironment,
-              let credential = appEnvironment.credential(forInstance: instance) else {
+        guard let appEnvironment else { return }
+
+        // The login can go between choosing this playlist and reading it —
+        // the merge dialog in particular waits on the user. Returning quietly
+        // would leave the row importing forever and let a batch carry on.
+        guard let credential = appEnvironment.credential(forInstance: instance) else {
+            loginChangedDuringImport = true
+            importingPlaylistID = nil
+            importProgress = nil
+            pendingMergePlaylist = nil
+            existingLocalPlaylist = nil
+            appEnvironment.toastManager.showError(
+                String(localized: "import.playlists.failed.title"),
+                subtitle: ImportError.loginChanged.localizedDescription
+            )
             return
         }
 
@@ -443,14 +459,22 @@ struct ImportPlaylistsView: View {
         for playlist in unimportedPlaylists {
             await addPlaylist(playlist)
 
-            // The rest of the batch belongs to a login that is no longer
-            // there. Reported once, by whichever import stopped first.
-            if loginChangedDuringImport { break }
-
             // If merge warning is shown, wait for user interaction
             while showMergeWarning {
                 try? await Task.sleep(for: .milliseconds(100))
             }
+
+            // The merge the dialog starts is detached from it, so dismissing
+            // the dialog is not the merge finishing. Waiting keeps the batch
+            // sequential — and lets a login change the merge runs into stop
+            // the rest, rather than the next import beginning under whoever
+            // replaced it.
+            await mergeImportTask?.value
+            mergeImportTask = nil
+
+            // The rest of the batch belongs to a login that is no longer
+            // there. Reported once, by whichever import stopped first.
+            if loginChangedDuringImport { break }
         }
     }