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_de.arb

@@ -597,6 +597,8 @@
   "tagManageAdd": "Tag hinzufügen",
   "tagManageRow": "{name} ({count})",
   "tagManageEdit": "Tag bearbeiten",
+  "tagManageHide": "Auf Karten ausblenden",
+  "tagManageShow": "Auf Karten zeigen",
   "tagManageDelete": "Tag löschen",
   "tagDeleteTitle": "Tag „{name}“ löschen?",
   "tagDeleteMessage": "{count, plural, =0{Er ist an keiner Karteikarte.} =1{Er wird auf all deinen Geräten von 1 Karteikarte entfernt.} other{Er wird auf all deinen Geräten von {count} Karteikarten entfernt.}}",