Starbucks Corp. plans to accelerate growth and brew up a stronger profit in the year ahead, with chief executive Howard Schultz noting that customers consider its drinks an ‘‘affordable luxury’’ even in challenging economic times. The Seattle company said global revenue at cafes open at least a year rose 6 percent in its fiscal fourth quarter, driven by higher customer traffic. Citing the strength of its core business and a pipeline of new products — such as improved baked goods and a single-serve brewer that was introduced this fall — Starbucks lifted its dividend and guidance for the fiscal year ending in September 2013.