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_de.arb

@@ -372,6 +372,9 @@
   "cardUnbury": "Zurückstellung aufheben",
   "cardTileBuried": "Karteikarte bis morgen zurückgestellt",
   "cardTileUnburied": "Karteikarte nicht mehr zurückgestellt",
+  "cardDeleteTitle": "Karteikarte löschen?",
+  "cardDeleteMessage": "Diese Karteikarte wird aus dem Lernset entfernt. Du kannst dies für einige Sekunden rückgängig machen.",
+  "cardDeleted": "Karteikarte gelöscht",
   "cardFilterTooltip": "Filter",
   "cardFilterTitle": "Filter",
   "cardFilterTag": "Tag",