Thankful city teens were gifted with the dresses of their dreams thanks to a big-hearted Hub e-commerce entrepreneur who donated beautiful, brand-new gowns to 21 lucky girls at the South Boston Boys & Girls Club.Sam Sisakhti, founder of the popular fashion website UsTrendy, recently launched “Believe in Yourself Project” — an initiative aimed at emphasizing the importance of a positive body image among young women — and asked the club to nominate girls between the ages of 13 and 17 who had shown strong academic growth and leadership qualities.