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/pages/Costs.tsx

@@ -153,9 +153,9 @@ export function Costs() {
                 </span>
               </p>
               {data.summary.budgetCents > 0 && (
-                <div className="w-full h-2 bg-muted rounded-full overflow-hidden">
+                <div className="w-full h-2 border border-border overflow-hidden">
                   <div
-                    className={`h-full rounded-full transition-[width,background-color] duration-150 ${
+                    className={`h-full transition-[width,background-color] duration-150 ${
                       data.summary.utilizationPercent > 90
                         ? "bg-red-400"
                         : data.summary.utilizationPercent > 70