feat(ui): add resource and usage dashboard (/usage route)

adds a new /usage page that lets board operators see how much each ai
provider is consuming across any date window, with per-model breakdowns,
rolling 5h/24h/7d burn windows, weekly budget bars, and a deficit notch
when projected spend is on track to exceed the monthly budget.

- new GET /companies/:id/costs/by-provider endpoint aggregates cost events
  by provider + model with pro-rated billing type splits from heartbeat runs
- new GET /companies/:id/costs/window-spend endpoint returns rolling window
  spend (5h, 24h, 7d) per provider with no schema changes
- QuotaBar: reusable boxed-border progress bar with green/yellow/red
  threshold fill colors and optional deficit notch
- ProviderQuotaCard: per-provider card showing budget allocation bars,
  rolling windows, subscription usage, and model breakdown with token/cost
  share overlays
- Usage page: date preset toggles (mtd, 7d, 30d, ytd, all, custom),
  provider tabs, 30s polling plus ws invalidation on cost_event
- custom date range blocks queries until both dates are selected and
  treats boundaries as local-time (not utc midnight) so full days are
  included regardless of timezone
- query key to timestamp is floored to the nearest minute to prevent
  cache churn on every 30s refetch tick

ui/src/App.tsx

@@ -19,6 +19,7 @@ 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";
@@ -147,6 +148,7 @@ 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 />} />