feat: keep undergraduate exams independent of course terms

mobile/lib/domain/app_snapshot.dart

@@ -28,8 +28,11 @@ class AppSnapshot {
     Map<String, String> calendarEventIds = const {},
   }) : settings = settings ?? AppSettings(),
        schoolTerm = schoolTerm ?? _legacyTerm(courses, examSchedule),
-       selectedSchool = selectedSchool ?? schoolTerm?.school ??
-           _legacyTerm(courses, examSchedule)?.school ?? examSchedule?.school,
+       selectedSchool =
+           selectedSchool ??
+           schoolTerm?.school ??
+           _legacyTerm(courses, examSchedule)?.school ??
+           examSchedule?.school,
        courses = List.unmodifiable(courses),
        calendarEventIds = Map.unmodifiable(calendarEventIds);
   final AppSettings settings;
@@ -75,13 +78,13 @@ class AppSnapshot {
     calendarEventIds: calendarEventIds ?? this.calendarEventIds,
     calendarCleanupPending:
         calendarCleanupPending ?? this.calendarCleanupPending,
-    selectedSchool:selectedSchool ?? this.selectedSchool,
+    selectedSchool: selectedSchool ?? this.selectedSchool,
   );
   Map<String, dynamic> toJson() => {
     'schemaVersion': 1,
     'examSchedule': examSchedule?.toJson(),
     'schoolTerm': schoolTerm?.toJson(),
-    'selectedSchoolId':selectedSchool?.id,
+    'selectedSchoolId': selectedSchool?.id,
     'courses': courses.map((c) => c.toJson()).toList(),
     'settings': {
       'semester': settings.semester == null
@@ -153,7 +156,9 @@ class AppSnapshot {
             );
       final metadata = json['importMetadata'];
       final snapshot = AppSnapshot(
-        selectedSchool:json['selectedSchoolId']==null ? null : SupportedSchool.fromId(json['selectedSchoolId'] as String),
+        selectedSchool: json['selectedSchoolId'] == null
+            ? null
+            : SupportedSchool.fromId(json['selectedSchoolId'] as String),
         schoolTerm: json['schoolTerm'] == null
             ? null
             : SchoolTerm.fromJson(
@@ -190,8 +195,9 @@ class AppSnapshot {
           (k, v) => MapEntry(k as String, v as String),
         ),
       );
-      if(snapshot.schoolTerm?.school != SupportedSchool.uestcUndergraduate &&
-          values.any((c)=>c.endPeriod>11)) throw const FormatException('第12节需要本科课表来源');
+      if (snapshot.schoolTerm?.school != SupportedSchool.uestcUndergraduate &&
+          values.any((c) => c.endPeriod > 11))
+        throw const FormatException('第12节需要本科课表来源');
       return snapshot;
     } catch (_) {
       throw const FormatException('保存文件内容无效，未覆盖已有数据');