Feat (i18n): danish language

[jesperordrup]

Summary

Adds Danish localization support to Bulwark Webmail so users can select Danish from the language switcher and use the app with the new da translation file.

Changes

• Added da to the supported locale routing configuration.
• Added Danish message loading for both server-side and client-side internationalization.
• Added Danish to the language switcher as Dansk.
• Added a Danish flag icon mapping for the language selector.
• Updated the internationalization feature summary to include Danish.

Related Issues

No related issue

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor / code quality improvement
• Chore / dependency update / CI change

Checklist

• I have read the Contributing Guide
• My code follows the project's code style and conventions
• I have run npm run typecheck && npm run lint and there are no errors
• The build passes (npm run build)
• I have tested my changes locally
• I have added or updated documentation if needed
• I have updated translations (locales/) if my changes affect user-facing text
• I have included screenshots or a screen recording for UI changes

Screenshots / Demo

Notes for Reviewers

[rathlinus]

Hi @jesperordrup
thanks for the translation. Just a few notes for next time:

Country codes for flags: name flag components by ISO country code (FlagDK), not language code, matches the existing convention (FlagGB for English, FlagBR for Portuguese, etc.).
camelCase SVG attributes: use strokeWidth instead of stroke-width to stay consistent with the rest of flag-icons.tsx.

thank you for your contribution