feat(app): lock the study screen, and undo the last rating

- Lock screen (header): a light scrim takes every touch; a tap shows
  Unlock for three seconds. Time locked doesn't count to the session's,
  and a running Pomodoro timer pauses, going on once unlocked (also
  when the session is left locked).
- Undo last rating (header): takes back one rating at a time within the
  round, its review deleted from the log, the round as it was; the card
  shows again from its question. Starting a round anew, or deleting a
  card, clears what can be undone.
- The review log deletes entries of any kind made at a moment (resets
  and ratings alike).

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

@@ -2656,6 +2656,30 @@ abstract class AppLocalizations {
   /// **'Close session'**
   String get studyClose;
 
+  /// No description provided for @studyUndoLastRating.
+  ///
+  /// In en, this message translates to:
+  /// **'Undo last rating'**
+  String get studyUndoLastRating;
+
+  /// No description provided for @studyLockScreen.
+  ///
+  /// In en, this message translates to:
+  /// **'Lock screen'**
+  String get studyLockScreen;
+
+  /// No description provided for @studyUnlock.
+  ///
+  /// In en, this message translates to:
+  /// **'Unlock'**
+  String get studyUnlock;
+
+  /// No description provided for @studyTapToUnlock.
+  ///
+  /// In en, this message translates to:
+  /// **'Tap to unlock'**
+  String get studyTapToUnlock;
+
   /// No description provided for @studyLoadFailed.
   ///
   /// In en, this message translates to: