A 30-year-old woman presents with a 2-year history of chronic widespread pain, fatigue, non-restorative sleep, and 'brain fog'. She reports multiple tender points on examination across her neck, shoulders, back, and hips. Extensive investigations including FBC, U&Es, LFTs, TFTs, CRP, ESR, RF, Anti-CCP, ANA, and CK are all normal. What is the most likely diagnosis?