feat(tag): a tag can be hidden on cards being studied

A tag's eye in Manage tags hides it on cards being studied, or shows it again: for the many tags naming where a word was met (an episode, a song), which would crowd a common word's card. A hidden tag still filters, narrows a study session and shows where tags are edited. Tag schema 2 adds 'hidden', written only for a hidden tag, so older apps go on reading every other.

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/tag/data/tag_payload_test.dart

@@ -20,6 +20,29 @@ void main() {
       });
     });
 
+    test('a hidden tag is schema 2 and says so; any other stays schema 1, '
+        'which older apps read', () {
+      final hidden = encodeTagPayload((
+        name: 'あかね噺 S1 E1',
+        colorName: 'fern',
+      ), hidden: true);
+      final shown = encodeTagPayload((name: 'あかね噺', colorName: 'fern'));
+
+      expect(decodePayload(hidden, newestSchema: tagSchemaVersion), {
+        'schemaVersion': 2,
+        'name': 'あかね噺 S1 E1',
+        'color': 'fern',
+        'hidden': true,
+      });
+      expect(tagPayloadHidden(hidden), isTrue);
+      expect(decodeTagPayload(hidden).name, 'あかね噺 S1 E1');
+      expect(
+        decodePayload(shown, newestSchema: tagSchemaVersion)['schemaVersion'],
+        1,
+      );
+      expect(tagPayloadHidden(shown), isFalse);
+    });
+
     test('reads a name as another device wrote it, tidied', () {
       final payload = encodePayload(1, {
         'name': ' Verb  form',
@@ -32,7 +55,7 @@ void main() {
     test('refuses a blank name, a newer schema and a blanked tag', () {
       for (final payload in [
         encodePayload(1, {'name': ' ', 'color': 'fern'}),
-        encodePayload(2, {'name': 'verb', 'color': 'fern'}),
+        encodePayload(3, {'name': 'verb', 'color': 'fern'}),
         redactedPayload,
       ]) {
         expect(() => decodeTagPayload(payload), throwsFormatException);

halkyon_learn_flutter/test/feature/tag/data/tag_repository_test.dart

@@ -67,6 +67,24 @@ void main() {
     });
   });
 
+  test('a tag hidden on cards stays hidden when renamed, and shows again '
+      'when asked; one is made hidden from the start if asked', () async {
+    final id = await repository.create('Verb', colorName: 'fern');
+
+    await repository.setHidden(id, hidden: true);
+    expect((await repository.read()).byId(id)?.hidden, isTrue);
+    await repository.update(id, name: 'Verbs', colorName: 'plum');
+    expect(
+      (await repository.read()).byId(id),
+      Tag(id: id, name: 'Verbs', colorName: 'plum', hidden: true),
+    );
+    await repository.setHidden(id, hidden: false);
+    expect((await repository.read()).byId(id)?.hidden, isFalse);
+
+    final episode = await repository.create('あかね噺 S1 E1', hidden: true);
+    expect((await repository.read()).byId(episode)?.hidden, isTrue);
+  });
+
   test('lists the tags by name, whatever their case', () async {
     await repository.create('verb');
     await repository.create('adjective');