feat(app): study sessions in rounds, by card type, cramming and reversed

A session studies a round of up to the settings' cards per round. In
spaced repetition a card leaves the round once it is a day or more away,
one rated lower comes back when due; cramming walks every card once,
round after round, its reviews moving no schedule, with going back and
on. Asking the back first (a toggle in the header, saved in the
settings) asks front/back and vocabulary cards their back, on their own
schedule. Each type studies its way: a word shows its reading and
meanings, a multiple-choice card is picked and checked, a matching card
sorted by dragging or tapping and checked; then the person rates (Bad,
OK, Good, Perfect, each with when the card comes back). Progress shows
as pills, or a tally bar while cramming; a round ends with how it went.
The session settings gain the mode, cards per round, new cards per day
and the order.

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

@@ -776,6 +776,172 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get cardTileConflict => 'Changed on two devices';
 
+  @override
+  String get studyClose => 'Close session';
+
+  @override
+  String get studyBackFirstOn =>
+      'Showing the back of cards first: tap to revert';
+
+  @override
+  String get studyBackFirstOff => 'Show the back of cards first';
+
+  @override
+  String get studyShowAnswer => 'Show answer';
+
+  @override
+  String get studyCheck => 'Check';
+
+  @override
+  String get studyRatingBad => 'Bad';
+
+  @override
+  String get studyRatingOk => 'OK';
+
+  @override
+  String get studyRatingGood => 'Good';
+
+  @override
+  String get studyRatingPerfect => 'Perfect';
+
+  @override
+  String get studyNothingTitle => 'Nothing to study right now';
+
+  @override
+  String get studyNothingBody => 'Come back later when more cards are due.';
+
+  @override
+  String studyNextDue(String interval) {
+    return 'The next card is due in $interval.';
+  }
+
+  @override
+  String get studyDone => 'Done';
+
+  @override
+  String get studyRoundDone => 'Round complete';
+
+  @override
+  String get studyContinue => 'Continue learning';
+
+  @override
+  String get studyPrevious => 'Previous card';
+
+  @override
+  String get studyNext => 'Next card';
+
+  @override
+  String get studyReading => 'Reading';
+
+  @override
+  String get studyMeanings => 'Meanings';
+
+  @override
+  String get studyPlaceItems =>
+      'Drag each item onto its category, or tap an item and then a category.';
+
+  @override
+  String studyMinutes(int count) {
+    return '$count min';
+  }
+
+  @override
+  String studyHours(int count) {
+    return '$count h';
+  }
+
+  @override
+  String studyDays(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count days',
+      one: '1 day',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String studyMonths(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count months',
+      one: '1 month',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String studyYears(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count years',
+      one: '1 year',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String studyFilteredBy(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'Narrowed to $count tags',
+      one: 'Narrowed to 1 tag',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get studySettingsCardsPerRound => 'Cards per round';
+
+  @override
+  String get studySettingsNewPerDay => 'New cards per day';
+
+  @override
+  String get studySettingsNewPerDayHint =>
+      'How many never-studied cards each studyset starts a day. Due reviews are never limited.';
+
+  @override
+  String get studySettingsUnlimited => 'Unlimited';
+
+  @override
+  String get studySettingsSortBy => 'Sort by';
+
+  @override
+  String get studySettingsLearnMode => 'Learn mode';
+
+  @override
+  String get studyModeSpaced => 'Spaced repetition';
+
+  @override
+  String get studyModeSpacedHint =>
+      'Due cards on their schedules; ratings move them.';
+
+  @override
+  String get studyModeCram => 'Cram';
+
+  @override
+  String get studyModeCramHint =>
+      'Walk every card without touching the schedules.';
+
+  @override
+  String get studySortOldestFirst => 'Oldest first';
+
+  @override
+  String get studySortRandom => 'Random';
+
+  @override
+  String get studySortNewestFirst => 'Newest first';
+
+  @override
+  String get studyFewer => 'Fewer';
+
+  @override
+  String get studyMore => 'More';
+
   @override
   String get commonCancel => 'Cancel';