feat(privacy): Finish takes in edits nobody signed once the person agrees (ADR 0019)

Finishing a partly readable studyset counts the edits a device without
the keys wrote readable while it was made private, by kind (flashcards,
notes, learning paths, PDFs, studysets), and asks: taken in, they are
sealed with the tree's key as the person's; left, they stay locked (the
short reading of 22 Oct, finding 11). In English and German. Tests on the
real gateways both ways, and of the dialog.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/test/support/fake_studyset_gateway.dart

@@ -77,6 +77,23 @@ class FakeStudysetGateway() implements StudysetGateway {
   @override
   bool keysMissing = false;
 
+  /// The unsigned edits each studyset holds, by kind.
+  final unsigned = <String, Map<String, int>>{};
+
+  /// The studysets whose unsigned edits were taken in.
+  final adopted = <String>[];
+
+  @override
+  Future<Map<String, int>> unsignedEditsOf(String id) async =>
+      unsigned[id] ?? const {};
+
+  @override
+  Future<int> adoptUnsignedEdits(String id) async {
+    adopted.add(id);
+    unsigned.remove(id);
+    return 0;
+  }
+
   @override
   Future<int> setPrivate(String id, {required bool private}) async {
     final tree = {id};