fix: SNMP status table no longer vanishes after back/return navigation (v0.11.9 Beta)

Root cause: backend filtered switches by status='online', so a mid-poll-cycle
device (briefly 'unknown') caused the endpoint to return [] — and the frontend
rendered nothing when the array was empty.

- Backend: query ALL switch devices (not just online ones); unreachable devices
  already fall through to the Promise.allSettled error path and render with an
  error badge, so no data is lost
- Frontend: replace silent `length > 0 &&` guard with a three-way ternary that
  shows a "No managed switches found" message when the array is empty, so the
  user always sees feedback instead of a blank section
- Frontend: add staleTime: 60_000 to the SNMP query to prevent the hair-trigger
  background refetch that raced with the poller on every navigation

frontend/src/pages/SwitchesSettingsPage.tsx

@@ -93,6 +93,7 @@ export default function SwitchesSettingsPage() {
   const { data: snmpStatuses = [], isLoading: snmpLoading, refetch: refetchSnmp, isFetching: snmpFetching } =
     useQuery({
       queryKey: ['switches-snmp'],
+      staleTime: 60_000,
       queryFn: () => switchesApi.getSnmpStatus().then(r => {
         // Pre-populate form from first successful result (if form not yet manually edited)
         const first = r.data.find(s => s.enabled != null && !s.error);
@@ -436,7 +437,9 @@ export default function SwitchesSettingsPage() {
           <div className="flex items-center gap-2 text-sm text-gray-400">
             <RefreshCw className="w-4 h-4 animate-spin" /> Checking SNMP status on all switches…
           </div>
-        ) : snmpStatuses.length > 0 && (
+        ) : snmpStatuses.length === 0 ? (
+          <p className="text-sm text-gray-400 dark:text-slate-500 py-1">No managed switches found.</p>
+        ) : (
           <div className="rounded-lg border border-gray-200 dark:border-slate-700 overflow-hidden">
             <table className="w-full text-sm">
               <thead>

frontend/src/version.ts

@@ -1 +1 @@
-export const APP_VERSION = 'v0.11.8 Beta';
+export const APP_VERSION = 'v0.11.9 Beta';