Replace hardcoded English strings across all 22 templates, 10 JS files,
and 4 locale files (en/fr/de/lb) with ~300 translation keys per language.
Uses server-side _() for Jinja2 templates and I18n.t() for JS toast
messages and dynamic Alpine.js expressions.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>