merge Usage page into Costs as two tabs ('Spend' and 'Providers'),
extract shared date-range logic to useDateRange() hook, delete /usage
route and sidebar entry, fix quota-windows bugs from prior review
reads local claude and codex auth files server-side, calls provider
quota apis (anthropic oauth usage, chatgpt wham/usage), and surfaces
live usedPercent per window in ProviderQuotaCard with threshold fill colors