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.dart

@@ -2938,11 +2938,11 @@ abstract class AppLocalizations {
   /// **'Card buried until tomorrow'**
   String get studyCardBuried;
 
-  /// No description provided for @studyUndoLastRating.
+  /// No description provided for @studyUndo.
   ///
   /// In en, this message translates to:
-  /// **'Undo last rating'**
-  String get studyUndoLastRating;
+  /// **'Undo'**
+  String get studyUndo;
 
   /// No description provided for @studyLockScreen.
   ///