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/components/Sidebar.tsx

@@ -4,6 +4,7 @@ import {
   Target,
   LayoutDashboard,
   DollarSign,
+  Gauge,
   History,
   Search,
   SquarePen,
@@ -107,6 +108,7 @@ export function Sidebar() {
         <SidebarSection label="Company">
           <SidebarNavItem to="/org" label="Org" icon={Network} />
           <SidebarNavItem to="/costs" label="Costs" icon={DollarSign} />
+          <SidebarNavItem to="/usage" label="Usage" icon={Gauge} />
           <SidebarNavItem to="/activity" label="Activity" icon={History} />
           <SidebarNavItem to="/company/settings" label="Settings" icon={Settings} />
         </SidebarSection>