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_de.dart

@@ -1539,6 +1539,18 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get studyClose => 'Sitzung schließen';
 
+  @override
+  String get studyUndoLastRating => 'Letzte Bewertung rückgängig machen';
+
+  @override
+  String get studyLockScreen => 'Bildschirm sperren';
+
+  @override
+  String get studyUnlock => 'Entsperren';
+
+  @override
+  String get studyTapToUnlock => 'Zum Entsperren tippen';
+
   @override
   String get studyLoadFailed => 'Diese Sitzung konnte nicht gelesen werden.';