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_en.arb

@@ -575,12 +575,6 @@
   "settingsDeleteMessage": "Everything you have, your studysets, cards and reviews, is deleted on the server and on all your devices. This can't be undone.",
   "settingsAbout": "About Halkyon Learn",
   "aboutLegalese": "Free software: the app is under the GNU Affero General Public License, version 3 or any later version, the server under the Server Side Public License, version 1.",
-  "settingsAppearance": "Appearance",
-  "settingsAppearanceDevice": "Device theme",
-  "settingsAppearanceLight": "Light",
-  "settingsAppearanceDark": "Dark",
-  "settingsLanguage": "Language",
-  "settingsLanguageDevice": "Device language",
   "settingsCitations": "Citations",
   "settingsCitationsDescription": "How sources and studysets' bibliographies are cited on this device. Type authors' full names: each style abbreviates and orders them itself.",
   "settingsCitationStyle": "Style",