feat: 主题切换

lib/models/common/theme_type.dart

@@ -0,0 +1,13 @@
+enum ThemeType {
+  light,
+  dark,
+  system,
+}
+
+extension ThemeTypeDesc on ThemeType {
+  String get description => ['浅色', '深色', '跟随系统'][index];
+}
+
+extension ThemeTypeCode on ThemeType {
+  int get code => [0, 1, 2][index];
+}