feat(app): choose whether studying shows the Pomodoro timer

Settings has a Study card: the Pomodoro profiles, and a switch "Show
the Pomodoro timer while studying" (on unless switched off). Like the
appearance, it is the device's own setting.

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_localizations_de.dart

@@ -1009,6 +1009,17 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get settingsCitationGuide => 'Offizieller Leitfaden';
 
+  @override
+  String get settingsStudy => 'Lernen';
+
+  @override
+  String get settingsStudyPomodoroTitle =>
+      'Pomodoro-Timer beim Lernen anzeigen';
+
+  @override
+  String get settingsStudyPomodoroSubtitle =>
+      'Zeige den Fokus-/Pausen-Countdown in der Lernsitzung, damit du die verbleibende Zeit siehst, ohne die Benachrichtigung zu öffnen.';
+
   @override
   String settingsVersion(String version, String build) {
     return 'Version $version (Build $build)';