feat(app): undo every step of a study session, deletions held until it ends

The header's Undo takes back ratings, burials and deletions, last first,
across the session's rounds and from the end of a round, and shows the
card again. A card deleted during a session is held (DelayedDeletions.hold)
until the session ends, then goes after the usual time to undo, so the
message offering Undo still works; if the app is killed first, the card
stays. Undo on a burial's or deletion's message takes back that step alone.

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_de.dart

@@ -1711,7 +1711,7 @@ class AppLocalizationsDe extends AppLocalizations {
   String get studyCardBuried => 'Karte bis morgen zurückgestellt';
 
   @override
-  String get studyUndoLastRating => 'Letzte Bewertung rückgängig machen';
+  String get studyUndo => 'Rückgängig machen';
 
   @override
   String get studyLockScreen => 'Bildschirm sperren';