A 28-year-old woman presents with progressive swelling of the face and legs over 3 weeks. She reports sudden-onset foamy urine and weight gain of 8 kg. On examination: BP 128/82 mmHg, bilateral pitting pedal edema, ascites. Laboratory findings: serum albumin 2.1 g/dL, total protein 4.8 g/dL, 24-hour urine protein 8.2 g, serum creatinine 0.9 mg/dL, blood urea 28 mg/dL. Lipid profile shows total cholesterol 320 mg/dL, LDL 240 mg/dL. Urinalysis: 4+ proteinuria, few RBCs, no casts. Renal ultrasound: normal-sized kidneys, normal echotexture. Serum complement C3 and C4 levels are normal. Which of the following is the most likely diagnosis?