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/lib/src/widget/undo_snack_bar.dart

@@ -1,27 +1,40 @@
+import 'dart:async';
+
 import 'package:halkyon_ui/l10n/halkyon_ui_localizations.dart';
 import 'package:material_ui/material_ui.dart';
 
 /// Says what just happened ([message]) and offers to undo it for three
 /// seconds; [onUndo] runs when the person takes that offer. A new message
-/// replaces the one showing.
+/// replaces the one showing. [onKept] runs once it has gone without Undo
+/// (timed out, or replaced by another), for what waits until it can no
+/// longer be undone, such as deleting for good.
 void showUndoSnackBar(
   BuildContext context, {
   required String message,
   required VoidCallback onUndo,
+  VoidCallback? onKept,
 }) {
   final l10n = HalkyonUiLocalizations.of(context);
-  ScaffoldMessenger.of(context)
-    ..hideCurrentSnackBar()
-    ..showSnackBar(
-      SnackBar(
-        content: Text(message),
-        // Shorter than the default four seconds, which feel slow for
-        // something that already happened.
-        duration: const Duration(seconds: 3),
-        // A snack bar with an action stays until dismissed unless told
-        // otherwise; this one goes, like any other.
-        persist: false,
-        action: SnackBarAction(label: l10n.undo, onPressed: onUndo),
-      ),
+  final messenger = ScaffoldMessenger.of(context)..hideCurrentSnackBar();
+  final shown = messenger.showSnackBar(
+    SnackBar(
+      content: Text(message),
+      // Shorter than the default four seconds, which feel slow for
+      // something that already happened.
+      duration: const Duration(seconds: 3),
+      // A snack bar with an action stays until dismissed unless told
+      // otherwise; this one goes, like any other.
+      persist: false,
+      action: SnackBarAction(label: l10n.undo, onPressed: onUndo),
+    ),
+  );
+  if (onKept != null) {
+    unawaited(
+      shown.closed.then((reason) {
+        if (reason != SnackBarClosedReason.action) {
+          onKept();
+        }
+      }),
     );
+  }
 }