feat: Add optional display name to user profile with corresponding API and UI updates

backend/src/models/user/user.rs

@@ -18,6 +18,9 @@ pub struct Model {
     /// Foreign key to roles table
     pub role_id: i32,
 
+    /// Display name (optional, separate from username)
+    pub name: Option<String>,
+
     // Profile fields
     /// Height in centimeters
     pub height_cm: Option<f32>,