fix(client): 修复 web 端选择文件后无反应问题

- 添加 result.files.isEmpty 检查（web file_picker 偶发空列表）
- web 平台不传 initialDirectory（避免潜在兼容性问题）
- 添加 mounted 检查和 try-catch，异常通过 SnackBar 显示

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/screens/settings/settings_screen.dart

@@ -1,5 +1,6 @@
 import 'package:dio/dio.dart';
 import 'package:file_picker/file_picker.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import 'package:flutter/services.dart';
@@ -1460,29 +1461,39 @@ class _BatchImportWidgetState extends ConsumerState<_BatchImportWidget> {
   }
 
   Future<void> _pickFile(int index) async {
-    final result = await FilePicker.platform.pickFiles(
-      type: FileType.custom,
-      allowedExtensions: ['xls', 'xlsx'],
-      withData: true,
-      initialDirectory: _lastDir,
-    );
-    if (result == null) return;
+    try {
+      final result = await FilePicker.platform.pickFiles(
+        type: FileType.custom,
+        allowedExtensions: ['xls', 'xlsx'],
+        withData: true,
+        initialDirectory: kIsWeb ? null : _lastDir,
+      );
+      if (result == null || result.files.isEmpty) return;
 
-    // 保存目录供下次使用
-    final path = result.files.first.path;
-    if (path != null) {
-      final dir = path.contains('/') ? path.substring(0, path.lastIndexOf('/')) : null;
-      if (dir != null) {
-        _lastDir = dir;
-        SharedPreferences.getInstance().then((p) => p.setString(_prefKey, dir));
+      // 保存目录供下次使用（仅非 web 平台）
+      if (!kIsWeb) {
+        final path = result.files.first.path;
+        if (path != null) {
+          final dir = path.contains('/') ? path.substring(0, path.lastIndexOf('/')) : null;
+          if (dir != null) {
+            _lastDir = dir;
+            SharedPreferences.getInstance().then((p) => p.setString(_prefKey, dir));
+          }
+        }
       }
-    }
 
-    setState(() {
-      _slots[index].file = result.files.first;
-      _slots[index].success = null;
-      _slots[index].error = null;
-    });
+      if (!mounted) return;
+      setState(() {
+        _slots[index].file = result.files.first;
+        _slots[index].success = null;
+        _slots[index].error = null;
+      });
+    } catch (e) {
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text('选择文件失败：$e')),
+      );
+    }
   }
 
   Future<void> _runImport() async {