fix: preserve queued settings and recover interrupted or corrupt state

mobile/lib/app/hepan_app.dart

@@ -159,6 +159,31 @@ class _ShellState extends State<_Shell> with WidgetsBindingObserver {
     }
   }
 
+  Future<void> _startFresh() async {
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (context) => AlertDialog(
+        title: const Text('重新建立课表'),
+        content: const Text(
+          '损坏文件会保留为恢复副本，然后重新建立空课表。已有课程通知会取消；手机日历中原有课程日程需手动处理。',
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(context, false),
+            child: const Text('取消'),
+          ),
+          FilledButton(
+            onPressed: () => Navigator.pop(context, true),
+            child: const Text('保留损坏文件并重新建立'),
+          ),
+        ],
+      ),
+    );
+    if (confirmed == true) {
+      await widget.controller.startFreshData();
+    }
+  }
+
   @override
   Widget build(BuildContext context) => AnimatedBuilder(
     animation: Listenable.merge([
@@ -198,6 +223,11 @@ class _ShellState extends State<_Shell> with WidgetsBindingObserver {
                                 onPressed: widget.controller.restoreBackup,
                                 child: const Text('恢复最近备份'),
                               ),
+                            if (!widget.controller.storageReady)
+                              TextButton(
+                                onPressed: _startFresh,
+                                child: const Text('重新建立课表'),
+                              ),
                           ],
                         ),
                       ),