feat(app): undo deleting a card

Deleting a card, from the list or while studying, asks first ("Delete
flashcard?"), then offers Undo for a few seconds ("Flashcard deleted").
Meanwhile the card leaves every list and session; once the time to
undo is over, its revisions are blanked and it is deleted for good, as
notes and studysets already are (CardDeletions, a DelayedDeletions).
Undo while studying puts the card back into the round where it was.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/src/app/app_router.dart

@@ -106,6 +106,7 @@ GoRouter buildRouter({String initialLocation = RoutePaths.home}) => GoRouter(
             create: (context) => StudySessionBloc(
               person.studysets,
               person.cards,
+              person.cardDeletions,
               person.reviews,
               person.tags,
               person.settings,