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/lib/l10n/app_localizations_de.dart

@@ -2159,6 +2159,12 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get tagManageEdit => 'Tag bearbeiten';
 
+  @override
+  String get tagManageHide => 'Auf Karten ausblenden';
+
+  @override
+  String get tagManageShow => 'Auf Karten zeigen';
+
   @override
   String get tagManageDelete => 'Tag löschen';