fix: 修复新增availability字段导致JSON解码失败

自定义init(from decoder:)，为新增字段提供默认值：
- availability默认.unknown
- 兼容旧版JSON数据，避免解码失败清空媒体库

Sources/MediaLibrary.swift

@@ -24,6 +24,11 @@ struct LibraryItem: Identifiable, Codable, Equatable {
     var logoURL: String?      // M3U tvg-logo
     var availability: StreamAvailability  // 流可用性
     
+    enum CodingKeys: String, CodingKey {
+        case id, url, name, type, lastPosition, liked, disliked, tags
+        case addedAt, group, logoURL, availability
+    }
+    
     init(id: String = UUID().uuidString, url: String, name: String,
          type: String = "url", lastPosition: Double = 0,
          liked: Bool = false, disliked: Bool = false,
@@ -43,6 +48,22 @@ struct LibraryItem: Identifiable, Codable, Equatable {
         self.availability = availability
     }
     
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        id = try container.decode(String.self, forKey: .id)
+        url = try container.decode(String.self, forKey: .url)
+        name = try container.decode(String.self, forKey: .name)
+        type = try container.decode(String.self, forKey: .type)
+        lastPosition = try container.decodeIfPresent(Double.self, forKey: .lastPosition) ?? 0
+        liked = try container.decodeIfPresent(Bool.self, forKey: .liked) ?? false
+        disliked = try container.decodeIfPresent(Bool.self, forKey: .disliked) ?? false
+        tags = try container.decodeIfPresent([String].self, forKey: .tags) ?? []
+        addedAt = try container.decode(Date.self, forKey: .addedAt)
+        group = try container.decodeIfPresent(String.self, forKey: .group)
+        logoURL = try container.decodeIfPresent(String.self, forKey: .logoURL)
+        availability = try container.decodeIfPresent(StreamAvailability.self, forKey: .availability) ?? .unknown
+    }
+    
     static func == (lhs: LibraryItem, rhs: LibraryItem) -> Bool { lhs.id == rhs.id }
 }