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>

Core/Entities/Student.cs

@@ -17,6 +17,10 @@ public class Student : IEquatable<Student>
     [Display(Name = "Last Name")]
     public string LastName { get; set; } = null!;
 
+    [StringLength(50)]
+    [Display(Name = "Nickname")]
+    public string? Nickname { get; set; }
+
     [Range(5,12)]
     [Display(Name = "Grade")]
     public int Grade { get; set;  }
@@ -55,6 +59,17 @@ public class Student : IEquatable<Student>
 
     public string FirstNameLastName => $"{FirstName} {LastName}";
 
+    /// <summary>
+    /// Nickname when set, otherwise <see cref="FirstName"/>.
+    /// </summary>
+    public string DisplayFirstName =>
+        string.IsNullOrWhiteSpace(Nickname) ? FirstName : Nickname.Trim();
+
+    public void NormalizeNickname()
+    {
+        Nickname = string.IsNullOrWhiteSpace(Nickname) ? null : Nickname.Trim();
+    }
+
     public static Tuple<string, string> ParseNameParts(string fullName)
     {
         var match = Match(fullName, @"(.*),\s*(.*)");
@@ -67,10 +82,7 @@ public class Student : IEquatable<Student>
                 : new Tuple<string, string>(fullName, string.Empty);
     }
 
-    public override string ToString()
-    {
-	    return FirstName;
-    }
+    public override string ToString() => DisplayFirstName;
 
     public bool VotingDelegate => OfficerRole is Entities.OfficerRole.President or Entities.OfficerRole.VicePresident;