themes and some refractoring

lib/src/models/srs_item.dart

@@ -0,0 +1,17 @@
+enum QuizMode { kanjiToEnglish, englishToKanji, reading, vocabToEnglish, englishToVocab, audioToEnglish }
+
+class SrsItem {
+  final int subjectId;
+  final QuizMode quizMode;
+  final String? readingType;
+  int srsStage;
+  DateTime lastAsked;
+
+  SrsItem({
+    required this.subjectId,
+    required this.quizMode,
+    this.readingType,
+    this.srsStage = 0,
+    DateTime? lastAsked,
+  }) : lastAsked = lastAsked ?? DateTime.now();
+}