feat: add undergraduate import and exam selection screens

mobile/lib/app/app_controller.dart

@@ -134,7 +134,9 @@ class AppController extends ChangeNotifier {
   Future<void> applyImport(
     ImportDraft draft, {
     required bool confirmedEmpty,
+    bool Function()? canApply,
   }) => _run(() async {
+    if (canApply != null && !canApply()) throw StateError('学校页面已变化，请重新导入。');
     if (draft.courses.isEmpty && (!draft.explicitlyEmpty || !confirmedEmpty)) {
       throw const FormatException('未检测到课程，已有课表保持不变');
     }
@@ -202,30 +204,32 @@ class AppController extends ChangeNotifier {
     }
     await _cleanupCalendar();
   });
-  Future<void> applyExams(ExamSchedule value) => _run(() async {
-    final validated = ExamSchedule.fromJson(value.toJson());
-    if (validated.school == SupportedSchool.uestcUndergraduate) {
-      await _save(snapshot.copyWith(examSchedule: validated));
-      return;
-    }
-    final term = SchoolTerm(int.parse(validated.termId));
-    final changed = _differentTerm(term);
-    await _save(
-      snapshot.copyWith(
-        examSchedule: validated,
-        schoolTerm: term,
-        courses: changed ? [] : null,
-        clearMetadata: changed,
-        settings: changed
-            ? snapshot.settings.copyWith(clearSemester: true)
-            : null,
-      ),
-    );
-    if (changed) {
-      isDemo = false;
-      await _reconcile();
-    }
-  });
+  Future<void> applyExams(ExamSchedule value, {bool Function()? canApply}) =>
+      _run(() async {
+        if (canApply != null && !canApply()) throw StateError('学校查询已取消，请重新读取。');
+        final validated = ExamSchedule.fromJson(value.toJson());
+        if (validated.school == SupportedSchool.uestcUndergraduate) {
+          await _save(snapshot.copyWith(examSchedule: validated));
+          return;
+        }
+        final term = SchoolTerm(int.parse(validated.termId));
+        final changed = _differentTerm(term);
+        await _save(
+          snapshot.copyWith(
+            examSchedule: validated,
+            schoolTerm: term,
+            courses: changed ? [] : null,
+            clearMetadata: changed,
+            settings: changed
+                ? snapshot.settings.copyWith(clearSemester: true)
+                : null,
+          ),
+        );
+        if (changed) {
+          isDemo = false;
+          await _reconcile();
+        }
+      });
   bool _differentTerm(SchoolTerm? next) =>
       next != null &&
       ((snapshot.schoolTerm != null && snapshot.schoolTerm != next) ||