Add a Language selector to Settings → Appearance that overrides the app
display language independent of the device locale. Persists a BCP-47 code
in ServerStoreState.appLanguage; MainActivity.attachBaseContext applies it
via a ContextWrapper (works on minSdk 26, no AppCompat dependency needed
since the Activity is a plain ComponentActivity). Selecting a language
recreates the activity so resources re-resolve immediately.
Includes System default / English / 한국어 options, plus a unit test for
the locale-code resolver.
Note: the new string keys (settings_item_language, language_system,
language_english, language_korean) should be backfilled into values-ko
once PR #602 lands.
Co-authored-by: hy4ri <hy4ri@users.noreply.github.com>
