A 34-year-old woman from Delhi presents with a 3-month history of palpitations, heat intolerance, and weight loss despite good appetite. On examination, she has a diffuse thyroid enlargement, fine tremor, and a resting heart rate of 110 bpm. Her TSH is <0.01 mIU/L (normal 0.4–4.0), free T4 is 28 pg/dL (normal 7.5–20), and free T3 is 8.5 pg/mL (normal 2.3–4.2). Thyroid peroxidase (TPO) antibodies and thyroglobulin antibodies are negative. Technetium-99m pertechnetate uptake is markedly elevated. What is the most likely diagnosis?