feat: add optional student nickname for informal display

Keep legal first and last names for formal lists; show DisplayFirstName on teams, calendars, and import matching so two Josiahs can be told apart.

Co-authored-by: Cursor <cursoragent@cursor.com>

Tests/Builders/StudentBuilder.cs

@@ -12,6 +12,7 @@ public class StudentBuilder
     private int _id = _idCounter++;
     private string _firstName = "Test";
     private string _lastName = "Student";
+    private string? _nickname = null;
     private int _grade = 9;
     private string? _email = null;
     private string? _phoneNumber = null;
@@ -41,6 +42,12 @@ public class StudentBuilder
         return this;
     }
 
+    public StudentBuilder WithNickname(string? nickname)
+    {
+        _nickname = nickname;
+        return this;
+    }
+
     public StudentBuilder WithGrade(int grade)
     {
         _grade = grade;
@@ -126,6 +133,7 @@ public class StudentBuilder
             Id = _id,
             FirstName = _firstName,
             LastName = _lastName,
+            Nickname = _nickname,
             Grade = _grade,
             Email = _email,
             PhoneNumber = _phoneNumber,