UFM: Update contentName component to respect current backoffice culture.

[engijlr]

Fixed: #20400

Problem

The umbContentName component in Block List labels always displayed content names in the default language, regardless of the currently active backoffice culture.

Solution

In content-name.element.ts, the code previously always accessed variants[0].name, which returned the first variant (typically the default language) regardless of the current backoffice culture. Now uses UmbDocumentItemDataResolver to get culture-aware content names for documents.

How to test

1. Create a document with multiple language variants.
2. Add a Block List with {umbContentName: combiGift} in the label
3. Switch between languages in the backoffice
4. Verify the block label updates to show the correct language variant

[leekelleher]

Tested out, working as expected! 🚀

I made a small tweak to set the default entity-type to UMB_DOCUMENT_ENTITY_TYPE instead of null, (this was a bug in my original code). As this is to support the Document Picker property-editor (which doesn't have a type on its value).