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/Services/HTMLParser.swift

@@ -93,10 +93,6 @@ enum HTMLParser {
         let h1 = try doc.select("h1").first()
         let fullTitle = try h1?.text().trimmingCharacters(in: .whitespacesAndNewlines) ?? "未知标题"
 
-        // 海报
-        let imgSrc = try doc.select("img.w-full.h-full.object-cover").first()?.attr("src") ?? ""
-        let posterURL = URL(string: imgSrc)
-
         // === 优先从 JSON-LD 提取结构化数据 ===
         var year = 0
         var rating: Double?
@@ -105,12 +101,17 @@ enum HTMLParser {
         var genres: [String] = []
         var description = ""
         var region = ""
+        var posterURL: URL?
 
         if let jsonLDScript = try doc.select("script[type=application/ld+json]").first() {
             let jsonText = try jsonLDScript.data()
             if let data = jsonText.data(using: .utf8),
                let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any] {
 
+                if let imageStr = json["image"] as? String {
+                    posterURL = URL(string: imageStr)
+                }
+
                 if let published = json["datePublished"] as? String {
                     year = Int(published) ?? 0
                 }
@@ -140,6 +141,12 @@ enum HTMLParser {
 
         // === 从 HTML 补充缺失数据 ===
 
+        // 海报 fallback: 主海报在 .flex-shrink-0 容器中
+        if posterURL == nil {
+            let imgSrc = try doc.select(".flex-shrink-0 img.object-cover").first()?.attr("src") ?? ""
+            posterURL = URL(string: imgSrc)
+        }
+
         // 评分 fallback
         if rating == nil {
             let ratingText = try doc.select(".rating-display").text()