A 25-year-old woman presents to a general neurology clinic with complaints of increasing clumsiness with heavy objects over the past year. She reports difficulty washing her hair and completing household chores such as hanging up laundry. On neurological examination, her upper limb power is graded using the MRC system. Her left arm has 3/5 power for shoulder abduction and 4/5 power for shoulder adduction, while her right arm has 3/5 power for shoulder abduction and 4/5 power for shoulder adduction. Reflexes are normal except for an inability to elicit the supinator reflex on the right side. Sensation is intact. Cranial nerve examination reveals drooping of the eyelids and decreased facial expression. Fatigability tests are unremarkable. What is the most likely diagnosis?