A 59-year-old woman presents with a rapidly enlarging, violaceous patch and several firm, purpuric nodules on the medial aspect of her right upper extremity. The lesions have developed over 3 months and occasionally bleed with minor trauma. She underwent a right modified radical mastectomy for breast cancer 8 years ago followed by adjuvant radiation therapy. Since surgery, she has had chronic, nonpitting swelling of the right arm that has progressively worsened. On examination, the affected arm is markedly edematous; overlying skin shows ecchymotic plaques with satellite papules. Which of the following is the most likely diagnosis?