feat(learn): the library on shelves by recency, narrowed to a subject (ADR 0041, D8)

The library as Vaia's (the developer's decisions of 10 Nov 2026): the
starred on a shelf of their own, then those studied or changed in the
last 7 days, the last month and before, each under its heading, the
archive last. Recent is the later of a card's last rating
(StudyProgress.lastStudied) and the studyset's own last change. A
dropdown narrows the library to one of the subjects in use, kept on the
device (DeviceSettings.librarySubject). A card names its subject under
its title, with its icon, and a language course its language beside.

Whats-New: The library groups your studysets by when you last used them, and narrows them to one subject.
Whats-New-De: Die Bibliothek ordnet deine Lernsets danach, wann du sie zuletzt genutzt hast, und zeigt auf Wunsch nur ein Fach.

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

apps/learn/halkyon_learn_flutter/test/feature/study/model/study_progress_test.dart

@@ -39,6 +39,46 @@ void main() {
     expect(progress.mastery, 0.25);
   });
 
+  test('names when a card among them was last rated, for the library to '
+      "shelve by (the developer's decision of 10 Nov 2026)", () {
+    CardSchedule ratedAt(DateTime at) => CardSchedule(
+      reviewCount: 1,
+      due: now.add(const Duration(days: 2)),
+      firstReviewedAt: at,
+      lastReviewedAt: at,
+    );
+    final progress = studyProgress(
+      groups: [
+        ['old', 'new'],
+        ['recent'],
+      ],
+      schedules: {
+        'old': ratedAt(DateTime.utc(2026, 9)),
+        'new': fresh,
+        'recent': ratedAt(DateTime.utc(2026, 10, 4)),
+      },
+      now: now,
+      startOfToday: startOfToday,
+    );
+
+    expect(progress.lastStudied, DateTime.utc(2026, 10, 4));
+    expect(
+      progress.withContent(notes: 1, paths: 0, files: 0).lastStudied,
+      DateTime.utc(2026, 10, 4),
+    );
+    expect(
+      studyProgress(
+        groups: [
+          ['new'],
+        ],
+        schedules: {'new': fresh},
+        now: now,
+        startOfToday: startOfToday,
+      ).lastStudied,
+      isNull,
+    );
+  });
+
   test('counts only as many new cards as today still starts', () {
     final order = [for (var i = 0; i < 30; i++) 'n$i'];