AssetManager 0.3.7 - Internationalization foundation

New:

English source/fallback language and German translation
Per-user language setting
Immediate language switch in navigation and profile
Language persists in database and session
Translation management under Settings > Translations
XLSX translation export/import
Missing-translation filter and direct editing
Existing users keep German; new users use configured default language (English by default)

No .env, config.json, database data, uploads or logs are included.