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_de.dart

@@ -6126,4 +6126,117 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get searchEngineCustomSave => 'Verwenden';
+
+  @override
+  String get studysetPartMaterial => 'Material';
+
+  @override
+  String get studysetPartProgress => 'Fortschritt';
+
+  @override
+  String get studysetPartPlan => 'Lernplan';
+
+  @override
+  String get studysetShowAll => 'Alle zeigen';
+
+  @override
+  String get studysetShowFewer => 'Weniger zeigen';
+
+  @override
+  String get progressLearned => 'Gelernt';
+
+  @override
+  String get progressLearnedInfo =>
+      'Karten, die du drei Wochen oder länger behältst.';
+
+  @override
+  String get progressOfStudyset => 'dieses Lernsets';
+
+  @override
+  String progressCardsLearned(int learned, int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$learned von $count Karten',
+      one: '$learned von 1 Karte',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get progressToReview => 'Zu wiederholen';
+
+  @override
+  String get progressToReviewInfo =>
+      'Schon gelernte Karten, die heute wieder fällig sind.';
+
+  @override
+  String progressDueToday(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'Karten heute fällig',
+      one: 'Karte heute fällig',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get progressStudyTime => 'Lernzeit';
+
+  @override
+  String get progressStudyTimeInfo =>
+      'Wie lange du die Karteikarten dieses Lernsets in den letzten sieben Tagen auf diesem Gerät gelernt hast.';
+
+  @override
+  String get progressThisWeek => 'diese Woche';
+
+  @override
+  String progressMinutes(int minutes) {
+    return '$minutes Min.';
+  }
+
+  @override
+  String progressHoursMinutes(int hours, int minutes) {
+    return '$hours Std. $minutes Min.';
+  }
+
+  @override
+  String planPathsDone(int done, int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$done von $count Pfaden abgeschlossen',
+      one: '$done von 1 Pfad abgeschlossen',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get planNoPaths => 'Noch keine Lernpfade';
+
+  @override
+  String get planNextUp => 'Als Nächstes';
+
+  @override
+  String planReview(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count Karteikarten lernen',
+      one: '1 Karteikarte lernen',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String planContinue(String title) {
+    return '„$title“ fortsetzen';
+  }
+
+  @override
+  String get planNothing => 'Gerade steht nichts an.';
+
+  @override
+  String get planLearningPaths => 'Lernpfade';
 }