Add English and French localization

[cerede2000]

Summary

• add complete English and French UI localization
• detect the browser language on first visit, persist the user choice, and fall back to English
• add English and French documentation with Docker Compose examples and a complete environment variable reference
• prepare multi-architecture publication to GHCR and Docker Hub

Validation

• frontend lint and typecheck
• frontend tests: 94 passed
• backend tests: 134 passed
• production frontend build
• Compose and GitHub Actions YAML validation
• manual browser validation in French and English