Dutch translations

[huulie]

1. Why is this change necessary?

This adds Dutch (nl) as a supported language for the UVdesk community skeleton 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

Follow-up TODOs:

• Make the customer OTP login JSON messages in uvdesk/support-center-bundle translatable instead of hardcoded English strings.
• Make the customer OTP login email subject translatable instead of hardcoded Login OTP from ....
• After those support-center-bundle changes, add the Dutch message keys for the OTP login success, validation, expiry, retry, and unknown-account states.
• Consider replacing user-facing OTP wording with clearer language such as login code / inlogcode.

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

Full language-picker support also needs the Dutch locale label in uvdesk/core-framework.
Companion core-framework PR: uvdesk/core-framework#722