San Francisco could become the nation’s leader in alternative energy use if voters approve two solar-energy ballot measures at the polls next week. Propositions B and H would enable the city to sell bonds to install solar panels on residential, commercial, and government rooftops, creating the largest solar power infrastructure in the United States. Advocates — including nearly every major civic organization and city officials of diverse political stripes — say the measures would reduce the city’s dependence on California’s unpredictable energy market and would be great for the environment.