feat(app,learn): the device's appearance and language, halkyon_app's, as Learn's Settings show them

The developer asked for Todo, Train and Meditate to offer Learn's
Appearance and Language (ADR 0014's amendment of 4 Nov 2026). Their
model, store, cubit and card move into halkyon_app: Appearance and Look
(a BCP 47 tag, null for the device's language), FileLookStore keeping it
in the app's support folder as JSON written by rename (per device, not
synced, read before the first frame), LookCubit for the apps to provide
above InstanceGate, and LookSettingsCard / DeviceLookCard with the texts
moved from Learn's arbs (look*, the same words). Each app passes the
languages it speaks, named in their own language.

Learn keeps its stored choice where it was, among its DeviceSettings,
and feeds the shared card from them: no migration, so nobody's language
or appearance moves, and nothing changes on its screen.

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

@@ -502,12 +502,6 @@
   "settingsDeleteMessage": "Alles, was du hast, also deine Lernsets, Karten und Wiederholungen, wird auf dem Server und auf all deinen Geräten gelöscht. Dies kann nicht rückgängig gemacht werden.",
   "settingsAbout": "Über Halkyon Learn",
   "aboutLegalese": "Freie Software: Die App steht unter der GNU Affero General Public License, Version 3 oder jeder späteren Version, der Server unter der Server Side Public License, Version 1.",
-  "settingsAppearance": "Darstellung",
-  "settingsAppearanceDevice": "Gerätedesign",
-  "settingsAppearanceLight": "Hell",
-  "settingsAppearanceDark": "Dunkel",
-  "settingsLanguage": "Sprache",
-  "settingsLanguageDevice": "Gerätesprache",
   "settingsCitations": "Quellenangaben",
   "settingsCitationsDescription": "Wie Quellen und die Literaturverzeichnisse von Lernsets auf diesem Gerät zitiert werden. Gib die vollständigen Namen der Autoren ein: Jeder Stil kürzt und ordnet sie selbst.",
   "settingsCitationStyle": "Stil",