Root cause: ember-intl merge order gives host app translations priority over
addon translations. The translations/en-us.yaml file was silently ignored.
Solution: Call intl.addTranslations('en-us', { 'app.name': 'Biibaye' })
at runtime from both setupExtension and onEngineLoaded hooks. This injects
the translation into the active intl service after the app boots.
Also removes the non-functional translations/en-us.yaml file.