feat: 文件存储安全加固 + 认证增强 + 媒体URL保护 + provider 重构

## 后端安全加固
- 新增 UserUploadPathResolver: 用户上传文件路径安全解析, 防目录穿越
- LocalReportFileStorage: 文件存储路径安全加固
- local_account_file_cleanup: 账号删除时文件清理逻辑增强
- AuthService: 认证逻辑增强
- file_endpoints / report_endpoints: 文件访问接口安全加固
- ai_chat_endpoints / doctor_endpoints: 接口安全调整
- Program.cs: 服务注册调整

## 前端认证与媒体
- 新增 authenticated_network_image.dart: 带认证的图片加载组件
- auth_provider: 认证状态管理大幅增强(+173)
- api_client: 网络客户端增强(+124)
- chat_provider: 聊天 provider 重构(+76)
- omron_device_provider: 蓝牙设备 provider 增强(+53)
- sse_handler: SSE 处理增强(+35)
- consultation_provider / data_providers / conversation_history_provider: 调整

## 页面调整
- remaining_pages: 健康档案/饮食记录等页面增强(+115)
- home_page / chat_messages_view: 主页微调
- doctor 端多页微调(consultations/dashboard/followups/patient_detail/profile/report_detail/reports)
- report_pages / settings_pages / notification_prefs_page: 微调
- device_scan_page / diet_capture_page / admin_home_page: 微调

## 测试
- 新增 file_path_security_tests: 文件路径安全测试
- 新增 protected_media_url_test: 媒体URL保护测试
- 新增 user_session_identity_test: 用户会话身份测试
- account_deletion_tests / application_service_tests / auth_tests: 更新

backend/tests/Health.Tests/account_deletion_tests.cs

@@ -13,8 +13,10 @@ public sealed class AccountDeletionTests
         var userId = Guid.NewGuid();
         var userDirectory = Path.Combine(root, "users", userId.ToString("N"));
         var reportPath = Path.Combine(root, "reports", "owned-report.pdf");
-        var chatImagePath = Path.Combine(root, "owned-chat.jpg");
-        var otherUserPath = Path.Combine(root, "users", Guid.NewGuid().ToString("N"), "keep.jpg");
+        var chatFileName = $"{Guid.NewGuid()}.jpg";
+        var chatImagePath = Path.Combine(userDirectory, chatFileName);
+        var otherUserId = Guid.NewGuid();
+        var otherUserPath = Path.Combine(root, "users", otherUserId.ToString("N"), "keep.jpg");
         var outsidePath = Path.Combine(Path.GetDirectoryName(root)!, "outside-account-file.txt");
 
         try
@@ -31,7 +33,7 @@ public sealed class AccountDeletionTests
             var cleanup = new LocalAccountFileCleanup(root);
             var references = new AccountFileReferences(
                 ["/uploads/reports/owned-report.pdf", "/uploads/../outside-account-file.txt"],
-                ["{\"imageUrl\":\"/uploads/owned-chat.jpg\"}"]);
+                [$"{{\"imageUrl\":\"/uploads/users/{userId:N}/{chatFileName}\"}}", $"{{\"imageUrl\":\"/uploads/users/{otherUserId:N}/keep.jpg\"}}"]);
 
             await cleanup.DeleteAsync(userId, references, CancellationToken.None);