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/YearTransition/YearTransitionPlanner_Tests.cs

@@ -196,6 +196,32 @@ public class YearTransitionPlanner_Tests
         Assert.That(result.UnmatchedNames, Is.EquivalentTo(new[] { "Nobody Here" }));
     }
 
+    [Test]
+    public void MatchPastedNames_MatchesNicknamePlusLastName()
+    {
+        var josiah = StudentBuilder.Create("Josiah", "Brown").WithNickname("Jo").WithGrade(7).Build();
+
+        var result = YearTransitionPlanner.MatchPastedNames(
+            [josiah],
+            ["Jo Brown", "Brown, Jo"]);
+
+        Assert.That(result.MatchedStudentIds, Is.EquivalentTo(new[] { josiah.Id }));
+        Assert.That(result.UnmatchedNames, Is.Empty);
+    }
+
+    [Test]
+    public void MatchPastedNames_LegalNameStillMatchesWhenNicknameSet()
+    {
+        var josiah = StudentBuilder.Create("Josiah", "Brown").WithNickname("Jo").WithGrade(7).Build();
+
+        var result = YearTransitionPlanner.MatchPastedNames(
+            [josiah],
+            ["Josiah Brown", "Brown, Josiah"]);
+
+        Assert.That(result.MatchedStudentIds, Is.EquivalentTo(new[] { josiah.Id }));
+        Assert.That(result.UnmatchedNames, Is.Empty);
+    }
+
     [Test]
     public void MatchPastedNames_AmbiguousWhenDuplicateNames()
     {