fix: extend APIError.statusCode to cover all cases

- unauthorized → 401
- serverError → extract code string as Int
- networkError → NSError.code

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

AIStudyApp/AIStudyApp/Core/Network/APIError.swift

@@ -14,8 +14,13 @@ enum APIError: LocalizedError {
 
     /// HTTP 状态码（如 401/403/404/500）。
     var statusCode: Int? {
-        if case .requestFailed(let code) = self { return code }
-        return nil
+        switch self {
+        case .requestFailed(let code): return code
+        case .unauthorized: return 401
+        case .serverError(_, let code): return code.flatMap { Int($0) }
+        case .networkError(let e): return (e as NSError).code
+        default: return nil
+        }
     }
 
     /// 语义错误码（如 AUTH_USER_DISABLED），用于 AppSession 状态判断