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/test/feature/studyset/bloc/studyset_activity_cubit_test.dart

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+import 'package:clock/clock.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/src/core/content/rich_content.dart';
+import 'package:halkyon_learn_flutter/src/feature/card/data/card_repository.dart';
+import 'package:halkyon_learn_flutter/src/feature/card/model/card_content.dart';
+import 'package:halkyon_learn_flutter/src/feature/study/data/review_repository.dart';
+import 'package:halkyon_learn_flutter/src/feature/study/model/rating.dart';
+import 'package:halkyon_learn_flutter/src/feature/study/model/review.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/bloc/studyset_activity_cubit.dart';
+
+import '../../../support/fake_card_gateway.dart';
+import '../../../support/fake_review_gateway.dart';
+
+void main() {
+  late CardRepository cards;
+  late ReviewRepository reviews;
+
+  setUp(() {
+    cards = CardRepository(FakeCardGateway());
+    reviews = ReviewRepository(FakeReviewGateway());
+    addTearDown(cards.dispose);
+    addTearDown(reviews.dispose);
+  });
+
+  Future<String> card(String studysetId) => cards.create(
+    studysetId,
+    CardContent(
+      front: RichContent.plain('猫'),
+      back: RichContent.plain('cat'),
+    ),
+  );
+
+  /// The first activity read for [studysetIds].
+  Future<StudysetActivityState> read(List<String> studysetIds) async {
+    final cubit = StudysetActivityCubit(cards, reviews, studysetIds);
+    addTearDown(cubit.close);
+    return (await cubit.stream.firstWhere((state) => state != null))!;
+  }
+
+  test('nothing studied yet: no studying, and no time this week', () async {
+    await card('japanese');
+
+    final activity = await read(['japanese']);
+
+    expect(activity.studied, isFalse);
+    expect(activity.week, List.filled(7, Duration.zero));
+  });
+
+  test('sums each day of the last seven, today last, across the studyset '
+      'and what is inside it; a fresh start is no studying', () async {
+    final now = DateTime(2026, 10, 12, 15);
+    final own = await card('japanese');
+    final inTopic = await card('grammar');
+    final elsewhere = await card('french');
+    Future<void> review(
+      String cardId,
+      DateTime at,
+      int minutes, {
+      StudyMode mode = StudyMode.review,
+    }) => reviews.record(
+      cardId: cardId,
+      rating: Rating.good,
+      reviewedAt: at,
+      duration: Duration(minutes: minutes),
+      mode: mode,
+    );
+    await review(own, now, 2);
+    await review(inTopic, now.subtract(const Duration(hours: 2)), 1);
+    await review(own, now.subtract(const Duration(days: 1)), 3);
+    await review(own, now.subtract(const Duration(days: 6)), 4);
+    // A week ago and more, another studyset's, a fresh start: none counts.
+    await review(own, now.subtract(const Duration(days: 7)), 5);
+    await review(elsewhere, now, 6);
+    await review(own, now, 7, mode: StudyMode.reset);
+
+    final activity = await withClock(
+      Clock.fixed(now),
+      () => read(['japanese', 'grammar']),
+    );
+
+    expect(activity.studied, isTrue);
+    expect(activity.week, [
+      const Duration(minutes: 4),
+      Duration.zero,
+      Duration.zero,
+      Duration.zero,
+      Duration.zero,
+      const Duration(minutes: 3),
+      const Duration(minutes: 3),
+    ]);
+  });
+
+  test('only a fresh start is no studying', () async {
+    final own = await card('japanese');
+    await reviews.record(
+      cardId: own,
+      rating: Rating.good,
+      reviewedAt: clock.now(),
+      duration: const Duration(minutes: 1),
+      mode: StudyMode.reset,
+    );
+
+    expect((await read(['japanese'])).studied, isFalse);
+  });
+}