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>

apps/learn/halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -2112,5 +2112,58 @@
   "workStagePacking": "Packen",
   "importAborted": "Import abgebrochen: Nichts davon wurde behalten.",
   "exportAborted": "Export abgebrochen.",
-  "exportProgressTitle": "Wird exportiert"
+  "exportProgressTitle": "Wird exportiert",
+  "subjectGroupSciences": "Naturwissenschaften und Gesundheit",
+  "subjectGroupComputing": "Mathematik und Informatik",
+  "subjectGroupSociety": "Gesellschaft",
+  "subjectGroupArts": "Sprachen und Künste",
+  "subjectGroupEveryday": "Alltag",
+  "subjectGroupOther": "Sonstiges",
+  "subjectBiology": "Biologie",
+  "subjectChemistry": "Chemie",
+  "subjectPhysics": "Physik",
+  "subjectAstronomy": "Astronomie",
+  "subjectEarthSciences": "Geowissenschaften",
+  "subjectEnvironmentalScience": "Umweltwissenschaften",
+  "subjectMedicine": "Medizin",
+  "subjectNursing": "Pflege",
+  "subjectPharmacy": "Pharmazie",
+  "subjectNutrition": "Ernährung",
+  "subjectPsychology": "Psychologie",
+  "subjectSportsScience": "Sportwissenschaft",
+  "subjectMathematics": "Mathematik",
+  "subjectStatistics": "Statistik",
+  "subjectComputerScience": "Informatik",
+  "subjectSoftwareEngineering": "Softwareentwicklung",
+  "subjectDataScienceAi": "Data Science und KI",
+  "subjectItOperations": "IT und Betrieb",
+  "subjectSecurityPrivacy": "Sicherheit und Datenschutz",
+  "subjectEngineering": "Ingenieurwesen",
+  "subjectArchitecture": "Architektur",
+  "subjectHistory": "Geschichte",
+  "subjectGeography": "Geografie",
+  "subjectPolitics": "Politik",
+  "subjectLaw": "Recht",
+  "subjectEconomics": "Volkswirtschaft",
+  "subjectBusiness": "Betriebswirtschaft",
+  "subjectMarketing": "Marketing",
+  "subjectAccountingFinance": "Rechnungswesen und Finanzen",
+  "subjectSociology": "Soziologie",
+  "subjectAnthropology": "Anthropologie",
+  "subjectEducation": "Pädagogik",
+  "subjectMediaStudies": "Medienwissenschaft",
+  "subjectPhilosophy": "Philosophie",
+  "subjectReligiousStudies": "Religionswissenschaft",
+  "subjectLanguages": "Sprachen",
+  "subjectLiterature": "Literatur",
+  "subjectLinguistics": "Linguistik",
+  "subjectArtDesign": "Kunst und Design",
+  "subjectMusic": "Musik",
+  "subjectTheatreFilm": "Theater und Film",
+  "subjectCookingHospitality": "Kochen und Gastronomie",
+  "subjectDrivingLicences": "Führerschein und Lizenzen",
+  "subjectOther": "Sonstiges / unsicher",
+  "studysetSheetSubject": "Fach",
+  "studysetSheetSubjectNone": "Fach wählen",
+  "subjectPickerTitle": "Fach"
 }