feat: persist timetable and validate confirmed imports

mobile/lib/data/demo_schedule.dart

@@ -0,0 +1,31 @@
+import '../domain/course.dart';
+
+List<Course> demoCourses() => [
+  _demo('english', '学术英语', '示例 · 张老师', '示例 · 综合楼 A301', 1, 1, 2),
+  _demo('matrix', '矩阵论', '示例 · 李老师', '示例 · 品学楼 B205', 1, 5, 6),
+  _demo('research', '研究方法', '示例 · 陈老师', '示例 · 立人楼 A208', 2, 3, 4),
+  _demo('signal', '现代信号处理', '示例 · 王老师', '示例 · 品学楼 C302', 3, 1, 2),
+  _demo('seminar', '学术研讨', '示例 · 刘老师', '示例 · 实验室 501', 3, 9, 11),
+  _demo('science', '科技论文写作', '示例 · 周老师', '示例 · 综合楼 B102', 4, 5, 6),
+  _demo('experiment', '实验与实践', '示例 · 赵老师', '示例 · 实验楼 302', 5, 3, 4),
+  _demo('weekend', '专题报告', '示例 · 导师组', '示例 · 学术报告厅', 7, 9, 11),
+];
+Course _demo(
+  String id,
+  String name,
+  String teacher,
+  String room,
+  int day,
+  int first,
+  int last,
+) => Course(
+  id: id,
+  name: name,
+  teacher: teacher,
+  room: room,
+  weekday: day,
+  startPeriod: first,
+  endPeriod: last,
+  weeks: Set.from(List.generate(20, (i) => i + 1)),
+  source: CourseSource.demo,
+);

mobile/lib/data/file_snapshot_store.dart

@@ -0,0 +1,49 @@
+import 'dart:io';
+import 'dart:convert';
+
+import 'snapshot_store.dart';
+import '../domain/app_snapshot.dart';
+
+class FileSnapshotStore extends SnapshotStore {
+  FileSnapshotStore(this.directory);
+  final Directory directory;
+  File get _file => File('${directory.path}/hepan_snapshot.json');
+  File get _backup => File('${directory.path}/hepan_snapshot.bak');
+  Future<AppSnapshot> _read(File file) async {
+    final value = jsonDecode(await file.readAsString());
+    if (value is! Map<String, dynamic>) throw const FormatException('保存文件损坏');
+    return AppSnapshot.fromJson(value);
+  }
+
+  @override
+  Future<AppSnapshot?> load() async =>
+      await _file.exists() ? await _read(_file) : null;
+  @override
+  Future<void> save(AppSnapshot snapshot) async {
+    await directory.create(recursive: true);
+    final text = jsonEncode(snapshot.toJson());
+    // Validate before touching the last successfully saved version.
+    AppSnapshot.fromJson(jsonDecode(text) as Map<String, dynamic>);
+    if (await _file.exists()) {
+      try {
+        await _read(_file);
+        await _file.copy(_backup.path);
+      } on FormatException {
+        /* Keep an existing valid backup. */
+      }
+    }
+    final temporary = File('${_file.path}.tmp');
+    await temporary.writeAsString(text, flush: true);
+    await temporary.rename(_file.path);
+  }
+
+  @override
+  Future<AppSnapshot> restoreBackup() async {
+    if (!await _backup.exists()) throw StateError('没有可恢复的备份');
+    final previous = await _read(_backup);
+    final temporary = File('${_file.path}.tmp');
+    await temporary.writeAsString(jsonEncode(previous.toJson()), flush: true);
+    await temporary.rename(_file.path);
+    return previous;
+  }
+}

mobile/lib/data/json_schedule_import.dart

@@ -0,0 +1,57 @@
+import '../domain/course.dart';
+
+import 'dart:convert';
+
+class ImportDraft {
+  const ImportDraft({
+    required this.courses,
+    required this.explicitlyEmpty,
+    required this.source,
+    required this.parserVersion,
+  });
+  final List<Course> courses;
+  final bool explicitlyEmpty;
+  final CourseSource source;
+  final String parserVersion;
+}
+
+class JsonScheduleImport {
+  static ImportDraft parse(String text) {
+    try {
+      if (utf8.encode(text).length > 2 * 1024 * 1024)
+        throw const FormatException('文件不能超过 2 MB');
+      final json = jsonDecode(text);
+      if (json is! Map ||
+          json['schemaVersion'] != 1 ||
+          json['courses'] is! List ||
+          (json['isDemo'] != null && json['isDemo'] is! bool) ||
+          (json['explicitlyEmpty'] != null &&
+              json['explicitlyEmpty'] is! bool)) {
+        throw const FormatException('请选择版本 1 的课表 JSON 文件');
+      }
+      final source = json['isDemo'] == true
+          ? CourseSource.demo
+          : CourseSource.file;
+      final courses = (json['courses'] as List)
+          .map(
+            (c) => Course.fromJson(
+              Map<String, dynamic>.from(c as Map),
+              source: source,
+            ),
+          )
+          .toList();
+      if (courses.map((c) => c.id).toSet().length != courses.length)
+        throw const FormatException('课程标识重复');
+      return ImportDraft(
+        courses: List.unmodifiable(courses),
+        explicitlyEmpty: json['explicitlyEmpty'] == true,
+        source: source,
+        parserVersion: 'json-v1',
+      );
+    } on FormatException {
+      rethrow;
+    } catch (_) {
+      throw const FormatException('课表文件格式无效');
+    }
+  }
+}

mobile/lib/data/snapshot_store.dart

@@ -0,0 +1,7 @@
+import '../domain/app_snapshot.dart';
+
+abstract class SnapshotStore {
+  Future<AppSnapshot?> load();
+  Future<void> save(AppSnapshot snapshot);
+  Future<AppSnapshot> restoreBackup() => Future.error(StateError('没有可恢复的备份'));
+}