Fix greeting header to use full name and truncate on small screens

What does this MR do and why?

Fix greeting header to use full name instead of first word only

Problem

The greeting header was splitting the user's full name on the first space and displaying only the first word. This caused issues for users with:

• Compound last names (e.g., "DE LA FONTAINE Jean" → "Hi, DE")
• Compound first names (e.g., "François Xavier DOE" → loses "Xavier")
• LDAP-synced names in "LASTNAME Firstname" format

Solution

Use the full name directly instead of splitting it. This is simpler and respects different naming conventions from various directory systems (LDAP, Active Directory, etc.).

References

Feedback → #553938 (comment 2932118869)

Resolves #577777 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Navigate to the homepage
2. Check for the full name being used in the greeting header

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