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_en.arb

@@ -413,6 +413,9 @@
   "cardUnbury": "Unbury",
   "cardTileBuried": "Flashcard buried until tomorrow",
   "cardTileUnburied": "Flashcard unburied",
+  "cardDeleteTitle": "Delete flashcard?",
+  "cardDeleteMessage": "This flashcard will be removed from the studyset. You can undo this for a few seconds.",
+  "cardDeleted": "Flashcard deleted",
   "cardFilterTooltip": "Filter",
   "cardFilterTitle": "Filter",
   "cardFilterTag": "Tag",