feat(app): appearance, language and version in Settings, and Settings from Home

Settings gets a card for how this device shows the app: its appearance
(as the device, light or dark) and its language (the device's, or one
the app speaks, each named in its own language), side by side from
medium windows on, as two outlined dropdowns. Both stay on the device
rather than following the person: they are kept in
<app support>/device_settings.json (written to a temporary file and
renamed into place), read before the app shows, so its first frame
already looks as chosen, and a file that can't be read falls back to
what the device does. DeviceSettingsBloc sits above SessionGate's keyed
subtree, since the settings outlive every person, and both the plain
app and the person's app take their theme mode and locale from it.

At the bottom of Settings, the release and its build, read once at
start together with the build the server is asked about.

Home has a Settings cog beside the streak, with the sync trouble dot as
on Profile. The library's create button reads "Create studyset", and
Delete is red in every menu that offers it: the studyset's card, screen
and rail, and the composer's draft.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -12,7 +12,7 @@
   "homeTitle": "Home",
 
   "libraryTitle": "Library",
-  "libraryNewStudyset": "New studyset",
+  "libraryCreateStudyset": "Create studyset",
   "libraryEmpty": "No studysets yet",
   "libraryUnreadable": "Couldn't read your studysets.",
   "libraryArchivedSection": "Archived",
@@ -267,6 +267,14 @@
   "settingsDeleteTitle": "Delete your account?",
   "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",
+  "settingsAppearance": "Appearance",
+  "settingsAppearanceDevice": "Device theme",
+  "settingsAppearanceLight": "Light",
+  "settingsAppearanceDark": "Dark",
+  "settingsLanguage": "Language",
+  "settingsLanguageDevice": "Device language",
+  "settingsVersion": "Version {version} (build {build})",
+  "@settingsVersion": {"placeholders": {"version": {"type": "String"}, "build": {"type": "String"}}},
   "syncTitle": "Sync",
   "syncStateSyncing": "Syncing…",
   "syncStateSynced": "Everything synced",