feat(learn): a studyset's subject, picked from a fixed list (ADR 0041)

A top studyset is filed under one of 44 subjects in six groups (the
developer's decisions of 9 and 10 Nov 2026): languages as one, computing
in five, Other / not sure last. Each has a Lucide icon and a default
colour, which picking it gives the studyset, its own to change from then
on. The studyset's sheet picks it (pickSubject); a sub-studyset shows its
top studyset's (subjectOf) and offers none. The payload keeps it as
`subject` (schema 8), so archives and published studysets carry it; an
ID this version doesn't know is kept and reads as none.

Whats-New: A studyset can now be filed under a subject, which gives it that subject's colour.
Whats-New-De: Ein Lernset lässt sich jetzt einem Fach zuordnen, das ihm dessen Farbe gibt.

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

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+# 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.
+- **Builds on:** ADR 0002 (payloads and their schemas), ADR 0005 (publishing), ADR 0031
+  (the palette's twenty colours).
+
+## Context
+
+A library of a hundred studysets, most of them computing books and language courses,
+shows them by title alone. Vaia files every studyset under a subject from a fixed
+list, shown on its card with a stripe in the subject's colour, and lets the library be
+narrowed to one.
+
+## Decisions
+
+- **D1.** One subject per studyset, from a fixed list of about Vaia's breadth, with
+  *Other / not sure* last.
+- **D2.** Each subject has a Lucide icon and a default colour from the palette. Choosing
+  one gives the studyset that colour, which stays the studyset's to change.
+- **D3.** Languages are one subject, *Languages*. The language itself comes from the
+  studyset's front language, which every course already gives.
+- **D4.** Computing is five subjects: *Computer science*, *Software engineering*, *Data
+  science and AI*, *IT and operations*, *Security and privacy*. Most of the library is
+  computing, and a single subject would narrow nothing.
+- **D5.** The subject sits on the top studyset alone, which its sub-studysets show as
+  theirs (`subjectOf`); their sheet offers none. It is kept in the studyset's sealed
+  payload as `subject`, a subject's ID (schema 8), and so goes into archives and is
+  published with the studyset, for Explore to filter by later.
+- **D6.** The list is the agent's draft of 10 Nov 2026, taken as it is (below).
+- **D7.** IDs are never renamed. An ID this app version doesn't know (a newer one's) is
+  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.
+- **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
+  column yet (it is in each release's repository).
+
+## The list (D6)
+
+ID, English and German name, Lucide icon, default colour (`Subject`, `subject.dart`).
+
+| Group | ID | English | German | Icon | Colour |
+|---|---|---|---|---|---|
+| Sciences and health | `biology` | Biology | Biologie | dna | fern |
+| | `chemistry` | Chemistry | Chemie | flask-conical | reef |
+| | `physics` | Physics | Physik | atom | slateblue |
+| | `astronomy` | Astronomy | Astronomie | telescope | enzian |
+| | `earth-sciences` | Earth sciences | Geowissenschaften | mountain | sahara |
+| | `environmental-science` | Environmental science | Umweltwissenschaften | leaf | forest |
+| | `medicine` | Medicine | Medizin | stethoscope | cerise |
+| | `nursing` | Nursing | Pflege | heart-pulse | strawberry |
+| | `pharmacy` | Pharmacy | Pharmazie | pill | pink |
+| | `nutrition` | Nutrition | Ernährung | apple | carrot |
+| | `psychology` | Psychology | Psychologie | brain | plum |
+| | `sports-science` | Sports science | Sportwissenschaft | dumbbell | copper |
+| Mathematics and computing | `mathematics` | Mathematics | Mathematik | sigma | cobalt |
+| | `statistics` | Statistics | Statistik | chart-column | ocean |
+| | `computer-science` | Computer science | Informatik | cpu | slateblue |
+| | `software-engineering` | Software engineering | Softwareentwicklung | code | enzian |
+| | `data-science-ai` | Data science and AI | Data Science und KI | brain-circuit | purple |
+| | `it-operations` | IT and operations | IT und Betrieb | server | pine |
+| | `security-privacy` | Security and privacy | Sicherheit und Datenschutz | shield-check | forest |
+| | `engineering` | Engineering | Ingenieurwesen | cog | soil |
+| | `architecture` | Architecture | Architektur | building-2 | sahara |
+| Society | `history` | History | Geschichte | landmark | soil |
+| | `geography` | Geography | Geografie | globe | pacific |
+| | `politics` | Politics | Politik | vote | cobalt |
+| | `law` | Law | Recht | scale | pickle |
+| | `economics` | Economics | Volkswirtschaft | trending-up | olive |
+| | `business` | Business | Betriebswirtschaft | briefcase | copper |
+| | `marketing` | Marketing | Marketing | megaphone | carrot |
+| | `accounting-finance` | Accounting and finance | Rechnungswesen und Finanzen | calculator | pine |
+| | `sociology` | Sociology | Soziologie | users | plum |
+| | `anthropology` | Anthropology | Anthropologie | footprints | sahara |
+| | `education` | Education | Pädagogik | school | ocean |
+| | `media-studies` | Media studies | Medienwissenschaft | tv | pink |
+| | `philosophy` | Philosophy | Philosophie | lightbulb | purple |
+| | `religious-studies` | Religious studies | Religionswissenschaft | book-open | olive |
+| Languages and arts | `languages` | Languages | Sprachen | languages | cerise |
+| | `literature` | Literature | Literatur | book-marked | strawberry |
+| | `linguistics` | Linguistics | Linguistik | speech | enzian |
+| | `art-design` | Art and design | Kunst und Design | palette | pink |
+| | `music` | Music | Musik | music | plum |
+| | `theatre-film` | Theatre and film | Theater und Film | clapperboard | cerise |
+| Everyday | `cooking-hospitality` | Cooking and hospitality | Kochen und Gastronomie | chef-hat | carrot |
+| | `driving-licences` | Driving and licences | Führerschein und Lizenzen | car | soil |
+| Other | `other` | Other / not sure | Sonstiges / unsicher | circle-question-mark | none |
+
+## Consequences
+
+- A studyset saved by this version is in schema 8, which older versions refuse to read,
+  as every schema before (ADR 0002): an archive with subjects imports from this version
+  on.
+- Adding a subject is cheap; renaming an ID isn't (D7), so names change, IDs don't.