Show a single progress bar representing aggregate progress across all
files in the queue. Useful for batch uploads with many files.
var taskProgress = {};
onTaskProgress: function(task, pct) {
taskProgress[task.id] = pct;
var sum = 0, count = 0;
for (var k in taskProgress) { sum += taskProgress[k]; count++; }
var avg = count > 0 ? sum / count : 0;
fill.style.width = avg + '%';
label.textContent = completed + ' of ' + total + ' (' + Math.round(avg) + '%)';
}