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_en.dart

@@ -1002,6 +1002,17 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get settingsCitationGuide => 'Official guide';
 
+  @override
+  String get settingsStudy => 'Study';
+
+  @override
+  String get settingsStudyPomodoroTitle =>
+      'Show the Pomodoro timer while studying';
+
+  @override
+  String get settingsStudyPomodoroSubtitle =>
+      'Show the focus / break countdown inside the study session, so you can see the time left without opening the notification.';
+
   @override
   String settingsVersion(String version, String build) {
     return 'Version $version (build $build)';