fix(dictionary): its sources in a short line, the full credits a tap away

The foot of the dictionary named each source with its makers and
licence, three lines of small print. It now names the sources alone in
a line ("Words from JMdict, JLPT vocabulary lists, Tatoeba"); tapping
it or its ⓘ shows who made each, under which licence, with its link,
which is still where the entries show, as the licences ask.

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

@@ -2364,6 +2364,9 @@ class AppLocalizationsDe extends AppLocalizations {
     return 'Wörter aus $sources.';
   }
 
+  @override
+  String get dictionarySourcesTitle => 'Quellen';
+
   @override
   String get dictionaryLookUp => 'Im Wörterbuch nachschlagen';