feat(app): ask what to study in a studyset with sub-studysets, and say when studying fails

- Studying a studyset that has sub-studysets and cards of its own
  first asks: everything (suggested) or this studyset only. Without
  either, it starts straight away. Alone, the back-first setting
  applies as usual and a turn of it is kept (a study button's focus
  may now leave the side to the settings).
- A session that can't be read says so, with Retry; a rating that
  can't be kept says so below the card, which stays to be rated
  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.dart

@@ -2656,6 +2656,60 @@ abstract class AppLocalizations {
   /// **'Close session'**
   String get studyClose;
 
+  /// No description provided for @studyLoadFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Couldn\'t read this session.'**
+  String get studyLoadFailed;
+
+  /// No description provided for @studyRatingFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Couldn\'t keep your rating. Rate the card again.'**
+  String get studyRatingFailed;
+
+  /// No description provided for @studyRetry.
+  ///
+  /// In en, this message translates to:
+  /// **'Retry'**
+  String get studyRetry;
+
+  /// No description provided for @studyScopeTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Study scope'**
+  String get studyScopeTitle;
+
+  /// No description provided for @studyScopeDescription.
+  ///
+  /// In en, this message translates to:
+  /// **'This studyset has sub-studysets. Pick which cards to review.'**
+  String get studyScopeDescription;
+
+  /// No description provided for @studyScopeEverythingTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Everything'**
+  String get studyScopeEverythingTitle;
+
+  /// No description provided for @studyScopeEverythingSubtitle.
+  ///
+  /// In en, this message translates to:
+  /// **'This studyset and all sub-studysets'**
+  String get studyScopeEverythingSubtitle;
+
+  /// No description provided for @studyScopeSelfTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'This studyset only'**
+  String get studyScopeSelfTitle;
+
+  /// No description provided for @studyScopeSelfSubtitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Skip cards in sub-studysets'**
+  String get studyScopeSelfSubtitle;
+
   /// No description provided for @studyBackFirstOn.
   ///
   /// In en, this message translates to: