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/ViewModels/DetailViewModel.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-@Observable
+@Observable @MainActor
 final class DetailViewModel {
     var detail: ContentDetail?
     var selectedSourceIndex = 0
@@ -27,23 +27,22 @@ final class DetailViewModel {
     func loadDetail(path: String) async {
         isLoading = true
         error = nil
-        defer { isLoading = false }
 
         do {
             let html = try await APIClient.shared.fetchDetailPage(path: path)
             let parsedDetail = try HTMLParser.parseContentDetail(html: html)
-            await MainActor.run {
-                self.detail = parsedDetail
-                self.selectedSourceIndex = 0
-                self.selectedEpisodeIndex = 0
-            }
+            self.detail = parsedDetail
+            self.selectedSourceIndex = 0
+            self.selectedEpisodeIndex = 0
         } catch is CancellationError {
+            // 被取消时不清除 loading 状态，让视图继续显示加载中
             return
         } catch let error as URLError where error.code == .cancelled {
             return
         } catch {
-            await MainActor.run { self.error = error.localizedDescription }
+            self.error = error.localizedDescription
         }
+        isLoading = false
     }
 
     func selectSource(_ index: Int) {