A 38-year-old male presents with progressive bilateral upper limb weakness, hand muscle atrophy, and fasciculations for 2 years. Neurological examination reveals mild atrophy in the deltoid, biceps, and forearm muscles, with marked atrophy of the thenar, hypothenar, and interosseous muscles bilaterally. Muscle strength: deltoid IV, elbow flexion/extension V, wrist joint IV, grip IV. Bilateral upper limbs show hypotonia. Biceps and triceps reflexes (±). The most likely diagnosis is: