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/YearTransition/YearTransitionPlanner.cs

@@ -242,7 +242,9 @@ public static class YearTransitionPlanner
         // Exact full-name matches first
         var fullMatches = students.Where(s =>
             comparer.Equals(s.FirstNameLastName, line) ||
-            comparer.Equals(s.LastNameFirstName, line)).ToList();
+            comparer.Equals(s.LastNameFirstName, line) ||
+            comparer.Equals($"{s.DisplayFirstName} {s.LastName}", line) ||
+            comparer.Equals($"{s.LastName}, {s.DisplayFirstName}", line)).ToList();
         if (fullMatches.Count > 0)
             return fullMatches;
 
@@ -251,8 +253,10 @@ public static class YearTransitionPlanner
             return [];
 
         return students.Where(s =>
-            comparer.Equals(s.FirstName.Trim(), first) &&
-            comparer.Equals(s.LastName.Trim(), last));
+            comparer.Equals(s.LastName.Trim(), last) &&
+            (comparer.Equals(s.FirstName.Trim(), first)
+             || comparer.Equals(s.DisplayFirstName, first)
+             || comparer.Equals(s.Nickname?.Trim(), first)));
     }
 
     /// <summary>