feat(costs): consolidate /usage into /costs with Spend + Providers tabs

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

ui/src/App.tsx

@@ -19,7 +19,6 @@ import { GoalDetail } from "./pages/GoalDetail";
 import { Approvals } from "./pages/Approvals";
 import { ApprovalDetail } from "./pages/ApprovalDetail";
 import { Costs } from "./pages/Costs";
-import { Usage } from "./pages/Usage";
 import { Activity } from "./pages/Activity";
 import { Inbox } from "./pages/Inbox";
 import { CompanySettings } from "./pages/CompanySettings";
@@ -148,7 +147,6 @@ function boardRoutes() {
       <Route path="approvals/all" element={<Approvals />} />
       <Route path="approvals/:approvalId" element={<ApprovalDetail />} />
       <Route path="costs" element={<Costs />} />
-      <Route path="usage" element={<Usage />} />
       <Route path="activity" element={<Activity />} />
       <Route path="inbox" element={<InboxRootRedirect />} />
       <Route path="inbox/recent" element={<Inbox />} />