Move every exposed UI string into res/values/strings.xml (+ plurals) and
read it via stringResource/pluralStringResource or getString, with 27
locale files carried over from the Flutter app's Weblate translations.
Only strings whose English is unchanged reuse an old translation; reworded
ones get fresh keys.
Persisted download failures become a DownloadError enum mapped to string
resources (stored by name, unknown legacy values degrade to Failed), the
per-app locale config is generated by AGP, and StringResourcesTest guards
locale keys and placeholders.
