feat(app): reset a studyset's progress, with an undo

The studyset's options offer Reset progress: after asking, every card
in it and in the studysets inside it starts afresh as new (a 'reset'
entry each), its history kept; an undo for a few seconds deletes those
entries again.

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

@@ -942,6 +942,22 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get studyMore => 'More';
 
+  @override
+  String get studysetMenuReset => 'Reset progress';
+
+  @override
+  String get studysetResetTitle => 'Reset progress?';
+
+  @override
+  String get studysetResetMessage =>
+      'Every flashcard in it, and in the studysets inside it, starts afresh as new, on all your devices. Your study history stays. You can undo this for a few seconds.';
+
+  @override
+  String get studysetResetConfirm => 'Reset';
+
+  @override
+  String get studysetResetDone => 'Progress reset';
+
   @override
   String get commonCancel => 'Cancel';