feat(studyset): Material, Progress and Study plan in place of the tabs

The studyset's four tabs become three parts, as Vaia has them, chosen
with a switch in the header that ticks as the navigation bar does: below
expanded windows in a line of its own under the title, above the path
and Topics; from there on at the end of the path's line, which the
topics leave free there.

- Material: the flashcards, notes and files one below the other, the
  first three of each with Show all; Browse, the tag filter (where all
  cards show, or while they are narrowed) and Reorder in each section's
  header; an outlined box where a section is empty.
- Progress, once anything in the studyset was studied (a card rated, a
  path finished): how much is learnt (mature cards), how many studied
  cards are due today, and how long it was studied this week on this
  device, with a line of the seven days (StudysetActivityCubit, from the
  reviews' durations).
- Study plan: a ring of the paths finished, Next up (study the cards due,
  go on with the first path not finished), then the paths.

It opens on the part it was left on; a tab kept before counts as its
part. Opened for the first time, it opens on the study plan while a path
is unfinished, else on the material. Material added lands in its part.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -6071,6 +6071,119 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get searchEngineCustomSave => 'Use';
+
+  @override
+  String get studysetPartMaterial => 'Material';
+
+  @override
+  String get studysetPartProgress => 'Progress';
+
+  @override
+  String get studysetPartPlan => 'Study plan';
+
+  @override
+  String get studysetShowAll => 'Show all';
+
+  @override
+  String get studysetShowFewer => 'Show fewer';
+
+  @override
+  String get progressLearned => 'Learned';
+
+  @override
+  String get progressLearnedInfo =>
+      'Cards you will remember for three weeks or more.';
+
+  @override
+  String get progressOfStudyset => 'of this studyset';
+
+  @override
+  String progressCardsLearned(int learned, int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$learned of $count cards',
+      one: '$learned of 1 card',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get progressToReview => 'To review';
+
+  @override
+  String get progressToReviewInfo =>
+      'Cards studied before that are due again today.';
+
+  @override
+  String progressDueToday(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'cards due today',
+      one: 'card due today',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get progressStudyTime => 'Study time';
+
+  @override
+  String get progressStudyTimeInfo =>
+      'How long you studied this studyset\'s flashcards on this device in the last seven days.';
+
+  @override
+  String get progressThisWeek => 'this week';
+
+  @override
+  String progressMinutes(int minutes) {
+    return '$minutes min';
+  }
+
+  @override
+  String progressHoursMinutes(int hours, int minutes) {
+    return '$hours h $minutes min';
+  }
+
+  @override
+  String planPathsDone(int done, int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$done of $count paths finished',
+      one: '$done of 1 path finished',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get planNoPaths => 'No learning paths yet';
+
+  @override
+  String get planNextUp => 'Next up';
+
+  @override
+  String planReview(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'Study $count flashcards',
+      one: 'Study 1 flashcard',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String planContinue(String title) {
+    return 'Continue “$title”';
+  }
+
+  @override
+  String get planNothing => 'Nothing due right now.';
+
+  @override
+  String get planLearningPaths => 'Learning paths';
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).