timetable performance optimization

lib/view/pages/timetable/data/timetable_appointment_factory.dart

@@ -213,10 +213,12 @@ class TimetableAppointmentFactory {
     return LessonColor.forStatus(status);
   }
 
-  McSubject? _findSubject(String? subjectShort) {
-    if (subjectShort == null) return null;
-    return subjects.where((s) => s.shortName == subjectShort).firstOrNull;
-  }
+  late final Map<String, McSubject> _subjectsByShort = {
+    for (final s in subjects.reversed) s.shortName: s,
+  };
+
+  McSubject? _findSubject(String? subjectShort) =>
+      subjectShort == null ? null : _subjectsByShort[subjectShort];
 
   String _subjectName(String? subjectShort, McTimetableEntry lesson) {
     if (subjectShort != null) {