Dutch translations

[huulie]

1. Why is this change necessary?

This adds Dutch (nl) as a supported language for the UVdesk community skeleton fork so Dutch-speaking agents and customers can use the helpdesk UI in their own language.

2. What does this change do, exactly?

• Adds translations/messages.nl.yml with Dutch translations based on the full English message catalogue.
• Registers nl in config/packages/uvdesk.yaml via app_locales.
• Adds the exact English catalogue key used by the local error page so that line can be translated as well.
• Keeps the change translation-focused: no default locale change, README badge update, or vendor/core-framework edits.

Translation choices:

• Formal/neutral Dutch tone using u where the UI addresses users directly.
• Generic helpdesk terminology for future upstream suitability, for example Customer = Klant, Agent = Medewerker, Knowledgebase = Kennisbank, and Saved Reply / Prepared Response = Standaardantwoord.
• Technical identifiers, placeholders, HTML tags, URLs, timezone IDs, product names, and API scopes are preserved.

Validation performed:

• ruby -e "require 'yaml'; YAML.load_file('translations/messages.en.yml'); YAML.load_file('translations/messages.nl.yml'); puts 'ok'"
• key parity/order check against translations/messages.en.yml (1623 entries each)
• placeholder/tag/URL preservation check
• local Twig |trans usage check: no missing Dutch keys
• git diff --check

Note: UVdesk's translation guide indicates that full language-picker support may also require adding the locale in uvdesk/core-framework's locale list. This PR only changes the community skeleton.

3. Please link to the relevant issues (if any).

None.

[huulie]

Closing as a duplicate of #922. PR #922 uses the clean upstream branch and contains the same Dutch translation changes without fork-only deployment commits.