Senate leaders this week will propose legislation to offer all workers in Massachusetts up to 12 weeks' paid time off to care for newborn and adopted children or sick family members, financed by an employee payroll premium of at least $1.50 a week. The bill, which would pay employees their full salary, up to $750 a week, would create the most generous paid leave policy in the nation. ''What family hasn't been touched by some emergency," said Senate President Robert Travaglini, who will formally unveil the proposal Tuesday before the Greater Boston Chamber of Commerce. ''I've been through physical and...