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

crates/pm-web/src/main.rs

@@ -288,6 +288,11 @@ pub fn build_router(state: AppState) -> Router {
             "/hosts/{host_id}/maintenance-windows",
             routes::maintenance_windows::router(),
         )
+        // Maintenance windows — bulk list-all endpoint
+        .nest(
+            "/maintenance-windows",
+            routes::maintenance_windows::all_windows_router(),
+        )
         // CA root certificate download
         .nest("/ca", routes::ca::ca_router())
         // Certificate list / renew / revoke