refactor(settings): 语言/密码/U盘导入改用右侧内嵌面板

- 新增 LanguagePanel / PasswordPanel / UsbImportPanel 三个 widget
- settings_page 移除 AlertDialog 弹窗逻辑
- 5 个菜单项统一走 _buildContent() switch 切换右侧内容
- 验证：flutter analyze 无新增 issue

lib/features/settings/widgets/password_panel.dart

@@ -0,0 +1,195 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../../../core/theme/app_theme.dart';
+import '../../../shared/services/toast_service.dart';
+import '../services/settings_service.dart';
+
+/// 密码修改面板
+///
+/// 在设置页右侧主区域渲染密码修改表单，行为与原弹窗一致：
+/// 校验空值、长度（≥6）、两次一致性、原密码正确性。
+class PasswordPanel extends ConsumerStatefulWidget {
+  const PasswordPanel({super.key});
+
+  @override
+  ConsumerState<PasswordPanel> createState() => _PasswordPanelState();
+}
+
+class _PasswordPanelState extends ConsumerState<PasswordPanel> {
+  final _oldCtrl = TextEditingController();
+  final _newCtrl = TextEditingController();
+  final _confirmCtrl = TextEditingController();
+  bool _submitting = false;
+  String? _error;
+
+  @override
+  void dispose() {
+    _oldCtrl.dispose();
+    _newCtrl.dispose();
+    _confirmCtrl.dispose();
+    super.dispose();
+  }
+
+  Future<void> _submit() async {
+    if (_submitting) return;
+    final oldPwd = _oldCtrl.text.trim();
+    final newPwd = _newCtrl.text.trim();
+    final confirmPwd = _confirmCtrl.text.trim();
+
+    if (oldPwd.isEmpty || newPwd.isEmpty || confirmPwd.isEmpty) {
+      setState(() => _error = '请填写所有字段');
+      return;
+    }
+    if (newPwd.length < 6) {
+      setState(() => _error = '新密码至少6位字符');
+      return;
+    }
+    if (newPwd != confirmPwd) {
+      setState(() => _error = '两次输入的新密码不一致');
+      return;
+    }
+
+    setState(() {
+      _submitting = true;
+      _error = null;
+    });
+
+    final isValid = await SettingsService.instance.verifyPassword(oldPwd);
+    if (!isValid) {
+      if (!mounted) return;
+      setState(() {
+        _submitting = false;
+        _error = '原密码错误';
+      });
+      return;
+    }
+
+    final ok = await SettingsService.instance.setPassword(newPwd);
+    if (!mounted) return;
+    setState(() => _submitting = false);
+
+    if (ok) {
+      _oldCtrl.clear();
+      _newCtrl.clear();
+      _confirmCtrl.clear();
+      ToastService.showSuccess(context, '密码已修改');
+    } else {
+      ToastService.showError(context, '密码修改失败');
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return ListView(
+      padding: EdgeInsets.zero,
+      children: [
+        Container(
+          padding: const EdgeInsets.all(16),
+          decoration: BoxDecoration(
+            color: Colors.white,
+            borderRadius: BorderRadius.circular(8),
+            border: Border.all(color: AppTheme.borderSubtle),
+          ),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.stretch,
+            children: [
+              Text(
+                '密码修改',
+                style: TextStyle(
+                  color: AppTheme.textPrimary,
+                  fontSize: 16,
+                  fontWeight: FontWeight.w600,
+                ),
+              ),
+              const Divider(),
+              _field(
+                controller: _oldCtrl,
+                label: '原密码',
+                hint: '请输入当前密码',
+              ),
+              const SizedBox(height: 16),
+              _field(
+                controller: _newCtrl,
+                label: '新密码',
+                hint: '至少6位字符',
+              ),
+              const SizedBox(height: 16),
+              _field(
+                controller: _confirmCtrl,
+                label: '确认新密码',
+                hint: '再次输入新密码',
+              ),
+              if (_error != null) ...[
+                const SizedBox(height: 12),
+                Text(
+                  _error!,
+                  style: TextStyle(color: AppTheme.errorColor, fontSize: 12),
+                ),
+              ],
+              const SizedBox(height: 20),
+              Row(
+                mainAxisAlignment: MainAxisAlignment.end,
+                children: [
+                  OutlinedButton(
+                    onPressed: _submitting
+                        ? null
+                        : () {
+                            _oldCtrl.clear();
+                            _newCtrl.clear();
+                            _confirmCtrl.clear();
+                            setState(() => _error = null);
+                          },
+                    child: const Text('重置'),
+                  ),
+                  const SizedBox(width: 12),
+                  ElevatedButton.icon(
+                    onPressed: _submitting ? null : _submit,
+                    icon: _submitting
+                        ? const SizedBox(
+                            width: 14,
+                            height: 14,
+                            child: CircularProgressIndicator(
+                              strokeWidth: 2,
+                              color: Colors.white,
+                            ),
+                          )
+                        : const Icon(Icons.check, size: 18),
+                    label: const Text('确认'),
+                  ),
+                ],
+              ),
+            ],
+          ),
+        ),
+        const SizedBox(height: 16),
+        Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 4),
+          child: Text(
+            '默认密码为 123456',
+            style: TextStyle(color: AppTheme.textSecondary, fontSize: 12),
+          ),
+        ),
+      ],
+    );
+  }
+
+  Widget _field({
+    required TextEditingController controller,
+    required String label,
+    required String hint,
+  }) {
+    return TextField(
+      controller: controller,
+      obscureText: true,
+      decoration: InputDecoration(
+        labelText: label,
+        hintText: hint,
+        border: const OutlineInputBorder(),
+        isDense: true,
+        contentPadding:
+            const EdgeInsets.symmetric(horizontal: 12, vertical: 12),
+      ),
+    );
+  }
+}