feat: select school terms and smooth refractive Dock motion

mobile/lib/domain/app_settings.dart

@@ -24,6 +24,7 @@ class AppSettings {
   final String? calendarId;
   AppSettings copyWith({
     Semester? semester,
+    bool clearSemester = false,
     bool? remindersEnabled,
     int? reminderMinutes,
     ColorMode? colorMode,
@@ -32,7 +33,7 @@ class AppSettings {
     String? calendarId,
     double? courseHeightScale,
   }) => AppSettings(
-    semester: semester ?? this.semester,
+    semester: clearSemester ? null : semester ?? this.semester,
     remindersEnabled: remindersEnabled ?? this.remindersEnabled,
     reminderMinutes: reminderMinutes ?? this.reminderMinutes,
     colorMode: colorMode ?? this.colorMode,

mobile/lib/domain/app_snapshot.dart

@@ -2,6 +2,7 @@ import 'app_settings.dart';
 import 'course.dart';
 import 'semester.dart';
 import 'exam_schedule.dart';
+import 'school_term.dart';
 
 class ImportMetadata {
   const ImportMetadata({
@@ -20,16 +21,32 @@ class AppSnapshot {
     List<Course> courses = const [],
     this.importMetadata,
     this.examSchedule,
+    SchoolTerm? schoolTerm,
     this.calendarCleanupPending = false,
     Map<String, String> calendarEventIds = const {},
-  }) : courses = List.unmodifiable(courses),
+  }) : schoolTerm = schoolTerm ?? _legacyTerm(courses, examSchedule),
+       courses = List.unmodifiable(courses),
        calendarEventIds = Map.unmodifiable(calendarEventIds);
   final AppSettings settings;
   final List<Course> courses;
   final ImportMetadata? importMetadata;
   final ExamSchedule? examSchedule;
+  final SchoolTerm? schoolTerm;
   final Map<String, String> calendarEventIds;
   final bool calendarCleanupPending;
+  static SchoolTerm? _legacyTerm(List<Course> courses, ExamSchedule? exams) {
+    final ids = courses
+        .map(
+          (course) => RegExp(r'^uestc-(\d+)-').firstMatch(course.id)?.group(1),
+        )
+        .whereType<String>()
+        .toSet();
+    if (exams != null) ids.add(exams.termId);
+    if (ids.length != 1) return null;
+    final id = int.tryParse(ids.single);
+    return id != null && id >= 1 && id <= 9999 ? SchoolTerm(id) : null;
+  }
+
   AppSnapshot copyWith({
     AppSettings? settings,
     List<Course>? courses,
@@ -38,6 +55,7 @@ class AppSnapshot {
     bool clearMetadata = false,
     ExamSchedule? examSchedule,
     bool clearExams = false,
+    SchoolTerm? schoolTerm,
     bool? calendarCleanupPending,
   }) => AppSnapshot(
     settings: settings ?? this.settings,
@@ -46,6 +64,7 @@ class AppSnapshot {
         ? null
         : importMetadata ?? this.importMetadata,
     examSchedule: clearExams ? null : examSchedule ?? this.examSchedule,
+    schoolTerm: schoolTerm ?? this.schoolTerm,
     calendarEventIds: calendarEventIds ?? this.calendarEventIds,
     calendarCleanupPending:
         calendarCleanupPending ?? this.calendarCleanupPending,
@@ -53,6 +72,7 @@ class AppSnapshot {
   Map<String, dynamic> toJson() => {
     'schemaVersion': 1,
     'examSchedule': examSchedule?.toJson(),
+    'schoolTerm': schoolTerm?.toJson(),
     'courses': courses.map((c) => c.toJson()).toList(),
     'settings': {
       'semester': settings.semester == null
@@ -116,6 +136,11 @@ class AppSnapshot {
             );
       final metadata = json['importMetadata'];
       return AppSnapshot(
+        schoolTerm: json['schoolTerm'] == null
+            ? null
+            : SchoolTerm.fromJson(
+                Map<String, dynamic>.from(json['schoolTerm'] as Map),
+              ),
         examSchedule: json['examSchedule'] == null
             ? null
             : ExamSchedule.fromJson(

mobile/lib/domain/school_term.dart

@@ -0,0 +1,27 @@
+/// The graduate system's term number, independent of teaching-week dates.
+class SchoolTerm {
+  SchoolTerm(this.id) {
+    if (id < 1 || id > 9999) throw ArgumentError('学校学期编号无效');
+  }
+  final int id;
+  String get label {
+    final year = 1990 + ((id - 1) ~/ 2);
+    return '$year-${year + 1}年${id.isOdd ? '秋' : '春'}';
+  }
+
+  static List<SchoolTerm> get choices =>
+      List.generate(73, (index) => SchoolTerm(73 - index));
+  Uri get timetableUri =>
+      Uri.https('yjsjy.uestc.edu.cn', '/pyxx/pygl/xskbcx/index/$id');
+  Uri get examUri =>
+      Uri.https('yjsjy.uestc.edu.cn', '/pyxx/pygl/kskcxx/index', {'xq': '$id'});
+  Map<String, dynamic> toJson() => {'id': id};
+  factory SchoolTerm.fromJson(Map<String, dynamic> json) {
+    if (json['id'] is! int) throw const FormatException('学校学期无效');
+    return SchoolTerm(json['id'] as int);
+  }
+  @override
+  bool operator ==(Object other) => other is SchoolTerm && other.id == id;
+  @override
+  int get hashCode => id.hashCode;
+}