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/test/support/fake_review_gateway.dart

@@ -37,13 +37,16 @@ class FakeReviewGateway() implements ReviewGateway {
   }
 
   @override
-  Future<void> deleteResets(Set<String> cardIds, DateTime at) async =>
-      rows.removeWhere(
-        (row) =>
-            cardIds.contains(row.cardId) &&
-            row.mode == 'reset' &&
-            row.reviewedAt == at,
-      );
+  Future<void> deleteReviews(
+    Set<String> cardIds,
+    DateTime at,
+    String mode,
+  ) async => rows.removeWhere(
+    (row) =>
+        cardIds.contains(row.cardId) &&
+        row.mode == mode &&
+        row.reviewedAt == at,
+  );
 
   @override
   Stream<void> get remoteChanges => const Stream.empty();