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_en.arb

@@ -674,6 +674,8 @@
   "tagManageRow": "{name} ({count})",
   "@tagManageRow": {"description": "A tag in the list of tags, with how many flashcards have it.", "placeholders": {"name": {"type": "String"}, "count": {"type": "int"}}},
   "tagManageEdit": "Edit tag",
+  "tagManageHide": "Hide on cards",
+  "tagManageShow": "Show on cards",
   "tagManageDelete": "Delete tag",
   "tagDeleteTitle": "Delete the tag '{name}'?",
   "@tagDeleteTitle": {"placeholders": {"name": {"type": "String"}}},