A 65-year-old female patient was admitted with purpura on both lower limbs for 1 month, accompanied by low-grade fever, pain in small joints of the hands, knees, and ankles (without swelling), and morning stiffness lasting about 10 minutes. Physical examination revealed multiple dental caries, with no joint deformity, swelling, or tenderness. Laboratory tests showed normal CBC, ANA 1:160 positive, RF 1:320 positive, anti-SSA antibody positive, and all other autoantibodies negative. Protein electrophoresis: γ-globulin 30%. The most likely diagnosis is: