feat(app): studyset payload schema 2, stars and the archive

A studyset is now its content (StudysetContent, everything the synced payload
holds) and the columns that change on their own: where it sits (parent,
position), and when it was archived or starred. Like a card's content, the
payload's part is edited as one value; the columns are written one by one,
so starring or archiving on one device and editing on another all survive a
sync (ADR 0004).

The payload's schema 2 adds the secondary title, the colour (a palette name),
the licence (SPDX identifier or name, holder, year) and the languages of the
cards' two sides (BCP 47), each left out when not given. Schema 1 reads as
schema 2 without them, and a field of the wrong kind makes the payload
unreadable rather than half read. Older app versions skip schema 2 studysets,
which costs nothing before the first release; fields foreseeable for the
first release's features are in, so none of them needs a schema 3 later.

The repository stars, unstars, archives and unarchives (at clock time, in
UTC), writing only that column. A new sync test (run against a server)
checks that a star and an archive on one device and a rename on another all
arrive on both.

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/feature/card/widget/card_widgets_test.dart

@@ -5,6 +5,7 @@ import 'package:halkyon_learn_flutter/src/feature/card/data/card_payload.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/data/card_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/model/card_content.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/data/studyset_payload.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset_content.dart';
 import 'package:material_ui/material_ui.dart' hide Card;
 
 import '../../../support/fake_card_gateway.dart';
@@ -51,10 +52,12 @@ void main() {
     final studysets = FakeStudysetGateway()
       ..receiveRemoteChange(
         'japanese',
-        encodeStudysetPayload((
-          title: 'Japanese',
-          description: RichContent.empty(),
-        )),
+        encodeStudysetPayload(
+          StudysetContent(
+            title: 'Japanese',
+            description: RichContent.empty(),
+          ),
+        ),
       );
     await pumpApp(
       tester,