fix: 修复详情页空白和海报图片错误

- DetailViewModel 添加 @MainActor 确保状态更新在主线程
- DetailView 消除空白初始状态，ProgressView 作为默认兜底
- 取消时保留加载状态避免页面闪回空白
- 使用 .task(id:) 确保切换条目时任务重新触发
- 海报优先从 JSON-LD image 字段获取，HTML fallback 改用正确选择器

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

DDYSClient/Views/Detail/DetailView.swift

@@ -12,22 +12,22 @@ struct DetailView: View {
 
     var body: some View {
         Group {
-            if viewModel.isLoading {
-                ProgressView()
-                    .frame(maxWidth: .infinity, maxHeight: .infinity)
-            } else if let error = viewModel.error {
-                errorView(error)
-            } else if let detail = viewModel.detail {
+            if let detail = viewModel.detail {
                 ScrollView {
                     detailContent(detail)
                 }
+            } else if let error = viewModel.error {
+                errorView(error)
+            } else {
+                ProgressView()
+                    .frame(maxWidth: .infinity, maxHeight: .infinity)
             }
         }
         .navigationTitle(item.title)
         #if os(iOS)
         .navigationBarTitleDisplayMode(.inline)
         #endif
-        .task {
+        .task(id: item.id) {
             if viewModel.detail == nil {
                 await viewModel.loadDetail(path: item.detailURL)
             }