Reports, split into Sprint and Overview

Reports was confusing: picking a sprint updated some charts but left others showing project-wide averages, so it was never clear what you were looking at. It's now two clear tabs.

• Sprint. Everything about one sprint and nothing else: scope, burndown, burnup, its status and type breakdown, and who's working on it. Change the sprint and the whole tab follows.
• Overview. Project-wide trends across every sprint: velocity, total workload, and the ticket mix. No sprint picker, because none of it is about a single sprint.
• Velocity that scales. The velocity chart now scrolls sideways once you have a lot of sprints, instead of squeezing them all into one view, and labels each sprint's completed points.