refactor(app): studysets and settings are written in the newest schema

Decided 2026-10-07: the codecs write the newest schema every time, as
before, rather than the oldest one that holds what is there. An older
app loses sight of a studyset or of the settings once a newer one has
written them, until it is updated too.

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/settings/model/person_settings_test.dart

@@ -181,11 +181,14 @@ void main() {
       expect(decodeSettingsPayload(payload), settings);
     });
 
-    test('as they come, are written in the third schema, which older apps '
-        'read', () {
-      final payload = encodeSettingsPayload(const PersonSettings());
+    test('as they come, are left out, in the fourth schema all the same', () {
+      final payload = decodePayload(
+        encodeSettingsPayload(const PersonSettings()),
+        newestSchema: 4,
+      );
 
-      expect(decodePayload(payload, newestSchema: 4)['schemaVersion'], 3);
+      expect(payload['schemaVersion'], 4);
+      expect(payload.keys, isNot(contains('pomodoro')));
     });
 
     test('that are malformed read as they come, keeping the other '