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/application_service_tests.cs

@@ -211,6 +211,24 @@ public sealed class ApplicationServiceTests
         Assert.Equal(["medication", "exercise", "followup"], events);
     }
 
+    [Fact]
+    public async Task Calendar_FollowUpUsesBeijingDateForUtcTimestamp()
+    {
+        var followUp = new FollowUp
+        {
+            Id = Guid.NewGuid(),
+            ScheduledAt = new DateTime(2026, 6, 18, 17, 0, 0, DateTimeKind.Utc),
+            Title = "北京时间复查"
+        };
+        var service = new CalendarService(new FakeCalendarRepository(
+            new CalendarDataSnapshot([], [], [followUp])));
+
+        var result = await service.GetMonthAsync(Guid.NewGuid(), 2026, 6, CancellationToken.None);
+
+        var target = Assert.Single(result);
+        Assert.Equal("2026-06-19", target.GetType().GetProperty("date")!.GetValue(target));
+    }
+
     [Fact]
     public async Task ExercisePlan_TenDays_CreatesTenUniqueConsecutiveDates()
     {