fix: resolve maintenance windows race condition and N+1 query

- Add GET /api/v1/maintenance-windows bulk endpoint to eliminate N+1
  per-host API calls (1 request instead of N+1)
- Fix two-phase state update race: setHosts() was called before
  setWindowsByHost(), causing React to render hosts with empty windows
- Add AbortController to cancel stale fetch requests on unmount/re-fetch
- Batch state updates atomically (React 18 auto-batching)
- Replace silent catch{} with proper error handling
- Add refreshData() wrapper for mutation handlers and Refresh button

Backend: maintenance_windows.rs - new list_all_windows handler +
all_windows_router(), mounted in main.rs
Frontend: client.ts - new listAll() API method
Frontend: MaintenanceWindowsPage.tsx - rewritten fetchData

frontend/src/api/client.ts

@@ -5,6 +5,7 @@ import type {
   CreateHostRequest,
   CreateJobRequest,
   CreateMaintenanceWindowRequest,
+  MaintenanceWindow,
   UpdateMaintenanceWindowRequest,
   Certificate,
   IssuedCert,
@@ -176,6 +177,10 @@ export const patchesApi = {
 
 // ── Maintenance Windows API ───────────────────────────────────────────────────
 export const maintenanceWindowsApi = {
+  /** Bulk: fetch ALL maintenance windows across every host in one request. */
+  listAll: () =>
+    apiClient.get<{ windows: MaintenanceWindow[] }>('/maintenance-windows'),
+  /** Per-host: fetch windows for a single host. */
   list: (hostId: string) =>
     apiClient.get(`/hosts/${hostId}/maintenance-windows`),
   create: (hostId: string, body: CreateMaintenanceWindowRequest) =>