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/l10n/app_localizations_en.dart

@@ -1311,6 +1311,16 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get cardTileUnburied => 'Flashcard unburied';
 
+  @override
+  String get cardDeleteTitle => 'Delete flashcard?';
+
+  @override
+  String get cardDeleteMessage =>
+      'This flashcard will be removed from the studyset. You can undo this for a few seconds.';
+
+  @override
+  String get cardDeleted => 'Flashcard deleted';
+
   @override
   String get cardFilterTooltip => 'Filter';