AssetManager 0.3.7 - Internationalization foundation

Stable rollback reference: 0.3.6.3 (German-only UI).

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.