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_en.dart

@@ -1528,6 +1528,36 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get studyClose => 'Close session';
 
+  @override
+  String get studyLoadFailed => 'Couldn\'t read this session.';
+
+  @override
+  String get studyRatingFailed =>
+      'Couldn\'t keep your rating. Rate the card again.';
+
+  @override
+  String get studyRetry => 'Retry';
+
+  @override
+  String get studyScopeTitle => 'Study scope';
+
+  @override
+  String get studyScopeDescription =>
+      'This studyset has sub-studysets. Pick which cards to review.';
+
+  @override
+  String get studyScopeEverythingTitle => 'Everything';
+
+  @override
+  String get studyScopeEverythingSubtitle =>
+      'This studyset and all sub-studysets';
+
+  @override
+  String get studyScopeSelfTitle => 'This studyset only';
+
+  @override
+  String get studyScopeSelfSubtitle => 'Skip cards in sub-studysets';
+
   @override
   String get studyBackFirstOn =>
       'Showing the back of cards first: tap to revert';