test: add locale consistency tests (web + mobile)
Adds locale consistency tests for both Vue frontend (Jest) and Flutter mobile:
Jest (web):
- Validates all locale files share the same top-level keys
- Each locale exports a valid object with content
- English locale always present
Flutter (mobile):
- Validates I18nService.supportedLanguages matches locale files on disk
- changeLanguage succeeds for every supported locale
- English locale always present
Bug fix: id.js (Indonesian) was missing the auth.errors section — caught by the new test.
These tests prevent future locale drift and ensure all translations stay in sync across languages.