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>

docs/adr/0041-learn-subjects.md

@@ -1,8 +1,8 @@
 # ADR 0041: Learn's subjects: one per studyset, from a fixed list
 
 - **Status:** decided 9 and 10 Nov 2026 by the developer: D1 to D4 and D6 are their own
-  choices, D5 and D7 to D9 the agent's recommendations, which they confirmed. D1 to D7
-  are built; D8 (the library) is next, and D10 later.
+  choices, D5 and D7 to D9 and D8's details the agent's recommendations, which they
+  confirmed. D1 to D9 are built; D10 comes later.
 - **Builds on:** ADR 0002 (payloads and their schemas), ADR 0005 (publishing), ADR 0031
   (the palette's twenty colours).
 
@@ -33,7 +33,17 @@ narrowed to one.
   kept as it is and reads as no subject until another is picked.
 - **D8.** The library, as Vaia's: a dropdown of the subjects in use, cards naming the
   subject under the title with a stripe in its colour, sections by recency (the last 7
-  days, the last month, older), the archive last.
+  days, the last month, older), the archive last. Decided on 10 Nov 2026:
+  - recent by the later of a card's last rating in the studyset (or inside it) and the
+    studyset's own last change; one from before changes were recorded counts from when
+    it was made, as its UUID v7 says (`lastUsedOf`, `shelvesOf`);
+  - the starred on a shelf of their own on top, however long ago they were used;
+  - the subject chosen kept on the device (`DeviceSettings.librarySubject`), all
+    subjects where none is chosen or the one kept is no longer in use; a studyset
+    without a subject, or with one this version doesn't know, is under *Other*;
+  - the stripe is the studyset's own colour, the card's bar on top, which the subject
+    sets when picked; the subject's line shows the subject's icon, and a language course
+    its language beside (*Languages · Japanese*).
 - **D9.** Existing studysets: the agent writes a subject into each of the developer's 72
   archives not yet imported; those already in the library get one by hand.
 - **D10.** Explore filters by subject later; its releases don't keep the subject in a