feat(library): Import and Refresh behind one menu button

The library's top bar had a button each for Import and Refresh. Pulling
the list down refreshes too, and is the quicker way on a phone, so both
now sit in a menu ("Library options"), leaving the bar to Search.

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

@@ -191,6 +191,9 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get libraryRefresh => 'Aktualisieren';
 
+  @override
+  String get libraryOptions => 'Optionen der Bibliothek';
+
   @override
   String get libraryImport => 'Importieren';