perf(learn): Delete now leaves Recently deleted at once

Deleting a studyset for good blanks and deletes every card, note, path
and PDF of its subtree before the studyset's row goes, which took about
3 s while the studyset stayed in Recently deleted. Now the repository's
Recently deleted leaves it out at once (StudysetRepository.leaving and
stay, applied to the value last read without reading again), the
deletion runs in the background, still one at a time in
StudysetDeletions' queue, and where it fails the studyset shows there
again, read anew with what is left of it, and a snack bar says so.

Whats-New: Delete now in Recently deleted takes a studyset away at once.
Whats-New-De: „Jetzt löschen“ unter „Zuletzt gelöscht“ entfernt ein Lernset sofort.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -36,6 +36,8 @@
   "libraryDeleteForGoodMessage": "„{title}“ wird samt allem, was darin ist, und allen zugehörigen Karten auf all deinen Geräten gelöscht. Das lässt sich nicht rückgängig machen.",
   "@libraryDeleteForGoodMessage": {"placeholders": {"title": {"type": "String"}}},
   "libraryDeleteForGoodConfirm": "Endgültig löschen",
+  "libraryDeleteForGoodFailed": "„{title}“ ließ sich nicht löschen. Es ist wieder unter „Zuletzt gelöscht“.",
+  "@libraryDeleteForGoodFailed": {"placeholders": {"title": {"type": "String"}}},
 
   "studysetSheetCreateTitle": "Lernset erstellen",
   "studysetSheetEditTitle": "Lernset bearbeiten",

apps/learn/halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -51,6 +51,13 @@
     }
   },
   "libraryDeleteForGoodConfirm": "Delete for good",
+  "libraryDeleteForGoodFailed": "“{title}” couldn't be deleted. It's back in Recently deleted.",
+  "@libraryDeleteForGoodFailed": {
+    "description": "A snack bar when deleting a studyset for good, which left Recently deleted at once, failed and it shows there again.",
+    "placeholders": {
+      "title": {"type": "String"}
+    }
+  },
 
   "studysetSheetCreateTitle": "Create studyset",
   "studysetSheetEditTitle": "Edit studyset",

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations.dart

@@ -280,6 +280,12 @@ abstract class AppLocalizations {
   /// **'Delete for good'**
   String get libraryDeleteForGoodConfirm;
 
+  /// A snack bar when deleting a studyset for good, which left Recently deleted at once, failed and it shows there again.
+  ///
+  /// In en, this message translates to:
+  /// **'“{title}” couldn\'t be deleted. It\'s back in Recently deleted.'**
+  String libraryDeleteForGoodFailed(String title);
+
   /// No description provided for @studysetSheetCreateTitle.
   ///
   /// In en, this message translates to:

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -115,6 +115,11 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get libraryDeleteForGoodConfirm => 'Endgültig löschen';
 
+  @override
+  String libraryDeleteForGoodFailed(String title) {
+    return '„$title“ ließ sich nicht löschen. Es ist wieder unter „Zuletzt gelöscht“.';
+  }
+
   @override
   String get studysetSheetCreateTitle => 'Lernset erstellen';
 

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -113,6 +113,11 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get libraryDeleteForGoodConfirm => 'Delete for good';
 
+  @override
+  String libraryDeleteForGoodFailed(String title) {
+    return '“$title” couldn\'t be deleted. It\'s back in Recently deleted.';
+  }
+
   @override
   String get studysetSheetCreateTitle => 'Create studyset';
 

apps/learn/halkyon_learn_flutter/lib/src/feature/studyset/data/studyset_deletions.dart

@@ -47,8 +47,20 @@ class StudysetDeletions(
   /// Brings the studyset [id] back from Recently deleted.
   Future<void> restore(String id) => _studysets.restore(id);
 
-  /// Deletes the studyset [id] for good, with everything inside it.
-  Future<void> deleteForGood(String id) => _queue.run(() => _deleteSubtree(id));
+  /// Deletes the studyset [id] for good, with everything inside it, which
+  /// takes a moment: Recently deleted leaves it out at once, and shows it
+  /// again where deleting it fails, with what is left of it; then this
+  /// throws as the deletion did.
+  Future<void> deleteForGood(String id) async {
+    _studysets.leaving(id);
+    try {
+      await _queue.run(() => _deleteSubtree(id));
+    } finally {
+      // Read again before it may show: gone, or as much of it as is left.
+      await _studysets.reread();
+      _studysets.stay(id);
+    }
+  }
 
   /// Deletes for good the studysets whose time in Recently deleted is over,
   /// now and whenever [checks] fires (a sync that went through, which may have

apps/learn/halkyon_learn_flutter/lib/src/feature/studyset/data/studyset_repository.dart

@@ -42,9 +42,41 @@ class StudysetRepository(final StudysetGateway _gateway) {
   Stream<List<Studyset>> watchAll() => _studysets.stream.map(notDeleted);
 
   /// The studysets waiting in Recently deleted ([recentlyDeleted]), now and
-  /// after every change.
-  Stream<List<Studyset>> watchDeleted() =>
-      _studysets.stream.map(recentlyDeleted);
+  /// after every change, but those being deleted for good ([leaving]).
+  Stream<List<Studyset>> watchDeleted() => _studysets.stream.map(
+    (all) => [
+      for (final studyset in recentlyDeleted(all))
+        if (!_leaving.contains(studyset.id)) studyset,
+    ],
+  );
+
+  /// The studysets being deleted for good right now, which Recently deleted
+  /// leaves out at once ([leaving]).
+  final _leaving = <String>{};
+
+  /// Leaves the studyset [id] out of Recently deleted at once, while it is
+  /// deleted for good, which takes a moment, until [stay] (optimistic
+  /// state, decided 2026-11-07).
+  void leaving(String id) {
+    if (_leaving.add(id)) {
+      _showAgain();
+    }
+  }
+
+  /// Shows the studyset [id] in Recently deleted again where it is still
+  /// there: deleting it for good is over, done or failed.
+  void stay(String id) {
+    if (_leaving.remove(id)) {
+      _showAgain();
+    }
+  }
+
+  /// Hands the studysets last read on again, for [_leaving] to apply.
+  void _showAgain() {
+    if (_studysets.value case final studysets? when !_studysets.isClosed) {
+      _studysets.add(studysets);
+    }
+  }
 
   /// Reads the studysets again for those following them ([watchAll]), as
   /// after a change: to try once more after a read failed.

apps/learn/halkyon_learn_flutter/lib/src/feature/studyset/widget/delete_studyset.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:developer';
 
 import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_learn_flutter/src/core/dependencies/person_dependencies.dart';
@@ -34,12 +35,15 @@ void deleteStudysetWithUndo(
 }
 
 /// Asks before deleting [studyset], waiting in Recently deleted, for good
-/// with everything inside it, which can't be undone; then does.
+/// with everything inside it, which can't be undone; then does, leaving
+/// Recently deleted at once while that runs in the background, and saying
+/// so where it fails, the studyset shown there again.
 Future<void> deleteStudysetForGood(
   BuildContext context,
   Studyset studyset,
 ) async {
   final l10n = AppLocalizations.of(context);
+  final messenger = ScaffoldMessenger.of(context);
   final deletions = PersonDependencies.of(context).deletions;
   if (await confirmDestructiveAction(
     context,
@@ -47,6 +51,14 @@ Future<void> deleteStudysetForGood(
     message: l10n.libraryDeleteForGoodMessage(studyset.content.title),
     confirmLabel: l10n.libraryDeleteForGoodConfirm,
   )) {
-    await deletions.deleteForGood(studyset.id);
+    final failed = l10n.libraryDeleteForGoodFailed(studyset.content.title);
+    unawaited(() async {
+      try {
+        await deletions.deleteForGood(studyset.id);
+      } on Object catch (error) {
+        log('Deleting ${studyset.id} for good failed', error: error);
+        messenger.showSnackBar(SnackBar(content: Text(failed)));
+      }
+    }());
   }
 }