feat(app): reset a studyset's progress, with an undo

The studyset's options offer Reset progress: after asking, every card
in it and in the studysets inside it starts afresh as new (a 'reset'
entry each), its history kept; an undo for a few seconds deletes those
entries again.

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

@@ -27,6 +27,15 @@ 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,
+      );
+
   @override
   Stream<void> get remoteChanges => const Stream.empty();
 }