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/lib/l10n/app_de.arb

@@ -2165,5 +2165,11 @@
   "subjectOther": "Sonstiges / unsicher",
   "studysetSheetSubject": "Fach",
   "studysetSheetSubjectNone": "Fach wählen",
-  "subjectPickerTitle": "Fach"
+  "subjectPickerTitle": "Fach",
+  "libraryAllSubjects": "Alle Fächer",
+  "librarySubjectTooltip": "Ein Fach zeigen",
+  "libraryShelfStarred": "Mit Stern",
+  "libraryShelfLastWeek": "Letzte 7 Tage",
+  "libraryShelfLastMonth": "Letzter Monat",
+  "libraryShelfOlder": "Älter"
 }