feat(study): a kanji's answer shows the studyset's words written with it

A character card's answer lists up to five vocabulary cards of the
studysets the session studies whose front holds the character, with
their reading and first meanings; a tap opens the word's card. The words
come from every card the session reads, whatever tags narrow it, so a
session of kanji alone still shows them, and nothing is fetched: it works
offline.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -5560,4 +5560,7 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get feedbackNeedsIdentity =>
       'Melde dich mit GitHub an, um im Original zu melden und vorzuschlagen.';
+
+  @override
+  String get kanjiExampleWords => 'Wörter damit';
 }