feat(todo): quick add's keys, sub-projects' colours, a task's menu on a right click, and the completed tasks deleted at once

The developer's list of 9 Nov 2026, Todo's part:

- Quick add: the down arrow and Tab move into the description, with its
  cursor (halkyon_editor's fix); Escape closes the projects or labels
  suggested first, then closes quick add, what was typed discarded; the
  project's chip shows the project's colour.
- A project without a colour of its own shows its parent's, the nearest
  ancestor's with one (colorOfProject, TodoCubit.colorOf), wherever a
  project's colour shows; the project editor's "Same colour as its parent"
  keeps it so, and the parent picker shows each project in its colour. A
  project of its own always has one.
- The task pane's x closes it, always; Close left the ⋮ menu.
- A task's row opens its menu on a right click: Mark as done, Date, Move
  to, Labels, Priority, Deadline, Duplicate and Delete, the selection's
  actions for that one task.
- Delete completed tasks, in the ⋮ of the Inbox, Today, Upcoming and a
  project, with Undo: the tasks leave the list at once and are deleted
  once the snack bar goes without its Undo (halkyon_ui's showUndoSnackBar
  gains onKept).

Whats-New: Quick add: the arrow keys and Tab reach the description, and Escape closes it without saving.
Whats-New-De: Schnell hinzufügen: Pfeiltasten und Tab erreichen die Beschreibung, und Escape schließt es, ohne zu speichern.
Whats-New: Sub-projects take their parent's colour unless they have their own.
Whats-New-De: Unterprojekte übernehmen die Farbe ihres übergeordneten Projekts, wenn sie keine eigene haben.
Whats-New: A right click on a task opens its menu, with Move to and more; Delete completed tasks clears a list, with Undo.
Whats-New-De: Ein Rechtsklick auf eine Aufgabe öffnet ihr Menü, mit Verschieben nach und mehr; Erledigte Aufgaben löschen räumt eine Liste auf, mit Rückgängig.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/test/widget/undo_snack_bar_test.dart

@@ -4,8 +4,12 @@ import 'package:material_ui/material_ui.dart';
 
 void main() {
   late int undos;
+  late int kept;
 
-  setUp(() => undos = 0);
+  setUp(() {
+    undos = 0;
+    kept = 0;
+  });
 
   /// Shows "Studyset archived" with a way to undo it.
   Future<void> archive(WidgetTester tester) async {
@@ -20,6 +24,7 @@ void main() {
                 context,
                 message: 'Studyset archived',
                 onUndo: () => undos++,
+                onKept: () => kept++,
               ),
               child: const Text('Archive'),
             ),
@@ -51,4 +56,25 @@ void main() {
     expect(find.text('Studyset archived'), findsNothing);
     expect(undos, 0);
   });
+
+  testWidgets('tells once it went without Undo, so what waited for it '
+      'happens then', (tester) async {
+    await archive(tester);
+    expect(kept, 0, reason: 'not while it shows');
+
+    await tester.pump(const Duration(seconds: 3));
+    await tester.pumpAndSettle();
+
+    expect(kept, 1);
+  });
+
+  testWidgets('never tells it was kept once undone', (tester) async {
+    await archive(tester);
+
+    await tester.tap(find.text('Undo'));
+    await tester.pumpAndSettle();
+    await tester.pump(const Duration(seconds: 5));
+
+    expect(kept, 0);
+  });
 }