feat(ios): 知识库列表卡片显示封面图

- ZLibraryCard 改用 coverUrl 显示封面图（56×56）
- 无封面时回退到默认书本图标
- KnowledgeBase 模型新增 coverUrl 字段

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AIStudyApp/AIStudyApp/Core/Models/APIModels.swift

@@ -164,6 +164,7 @@ struct KnowledgeBase: Codable, Identifiable {
     let title: String
     let description: String?
     let coverKey: String?
+    let coverUrl: String?
     let status: String?
     let itemCount: Int?
     let lastStudiedAt: String?

AIStudyApp/AIStudyApp/Features/Library/LibraryHomeView.swift

@@ -36,7 +36,7 @@ struct LibraryHomeView: View {
                     }
                     ForEach(viewModel.knowledgeBases) { kb in
                         NavigationLink(value: Route.libraryDetail(knowledgeBaseId: kb.id)) {
-                            ZLibraryCard(icon: "books.vertical.fill", name: kb.title, desc: kb.description ?? "", color: Color.zxPurple, items: kb.itemCount ?? 0, mastery: 50, tags: [], last: lastStudiedText(kb.lastStudiedAt))
+                            ZLibraryCard(coverUrl: kb.coverUrl, name: kb.title, desc: kb.description ?? "", items: kb.itemCount ?? 0, last: lastStudiedText(kb.lastStudiedAt))
                         }
                     }
                     if viewModel.knowledgeBases.isEmpty && !viewModel.isLoading {
@@ -59,8 +59,33 @@ struct LibraryHomeView: View {
         return iso.prefix(10).description
     }
 }
-struct ZLibraryCard: View { let icon: String; let name: String; let desc: String; let color: Color; let items: Int; let mastery: Int; let tags: [String]; let last: String
+struct ZLibraryCard: View { let coverUrl: String?; let name: String; let desc: String; let items: Int; let last: String
-    var body: some View { VStack(spacing: 0) { Rectangle().fill(ZXGradient.progressBar).frame(height: 3); HStack(spacing: 12) { Image(systemName: icon).font(.system(size: 20)).foregroundColor(color).frame(width: 44, height: 44).background(color.opacity(0.12)).clipShape(RoundedRectangle(cornerRadius: 13)).overlay(RoundedRectangle(cornerRadius: 13).stroke(color.opacity(0.3), lineWidth: 1)); VStack(alignment: .leading, spacing: 2) { Text(name).font(.system(size: 16, weight: .bold)).foregroundColor(Color.zxF0); Text(desc).font(.system(size: 12)).foregroundColor(Color.zxF04); Text("掌握 \(mastery)%").font(.system(size: 11)).foregroundColor(Color.zxF04) }; Spacer() }.padding(16); HStack { HStack(spacing: 4) { Image(systemName: "clock").font(.system(size: 10)); Text("\(items) 项 · \(last)").font(.system(size: 11)) }.foregroundColor(Color.zxF03); Spacer(); ForEach(tags.prefix(2), id: \.self) { t in Text(t).font(.system(size: 10, weight: .medium)).foregroundColor(Color.zxPurple).padding(.horizontal, 7).padding(.vertical, 2).background(Color(hex: "#7C6EFA", opacity: 0.08)).clipShape(Capsule()) } }.padding(.horizontal, 16).padding(.bottom, 12) }
+    var body: some View { VStack(spacing: 0) {
+        HStack(spacing: 12) {
+            // 封面图
+            ZStack {
+                RoundedRectangle(cornerRadius: 13).fill(Color.zxPurpleBG(0.12)).frame(width: 56, height: 56)
+                if let url = coverUrl, let imageUrl = URL(string: url) {
+                    AsyncImage(url: imageUrl) { phase in
+                        switch phase {
+                        case .success(let img): img.resizable().scaledToFill().frame(width: 56, height: 56).clipShape(RoundedRectangle(cornerRadius: 13))
+                        default: Image(systemName: "books.vertical.fill").font(.system(size: 22)).foregroundColor(Color.zxPurple.opacity(0.5))
+                        }
+                    }
+                } else {
+                    Image(systemName: "books.vertical.fill").font(.system(size: 22)).foregroundColor(Color.zxPurple.opacity(0.5))
+                }
+            }
+            VStack(alignment: .leading, spacing: 4) {
+                Text(name).font(.system(size: 16, weight: .bold)).foregroundColor(Color.zxF0)
+                if !desc.isEmpty { Text(desc).font(.system(size: 12)).foregroundColor(Color.zxF04).lineLimit(1) }
+            }
+            Spacer()
+        }.padding(16)
+        HStack {
+            HStack(spacing: 4) { Image(systemName: "clock").font(.system(size: 10)); Text("\(items) 项 · \(last)").font(.system(size: 11)) }.foregroundColor(Color.zxF03)
+            Spacer()
+        }.padding(.horizontal, 16).padding(.bottom, 12) }
         .background(Color.zxFill003).clipShape(RoundedRectangle(cornerRadius: 20)).overlay(RoundedRectangle(cornerRadius: 20).stroke(Color.zxBorder006, lineWidth: 1)) }
 }