Hawaii enjoyed the biggest boom in sales during the first three months of the year, a giant 57.9% increase, the National Assn. of Realtors reported. Nationally, sales of existing single-family homes, town houses and condominiums climbed to an annual rate of 4.01 million units. The association said the lowest mortgage rates in nine years spurred sales during the January-March quarter. The biggest increase in prices occurred in the Northeast, led by a 40.9% jump in home prices in Providence, R.I., where the median price for an existing home was $101,300, compared to $71,900 in first quarter 1986. In Los Angeles, the median price was $130,100, up 8.1% from a year ago. Nationally, the median price of an existing home rose 7.4% to $83,900.