fix(ui): add Outlet to settings layout for child route navigation

Settings child routes (credentials, snmp-profiles, api-keys) were not
rendering because the parent settings.tsx had no Outlet. Now detects
child routes and renders Outlet, or shows SettingsPage index.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/_authenticated/settings.tsx

@@ -1,6 +1,21 @@
-import { createFileRoute } from '@tanstack/react-router'
+import { createFileRoute, Outlet, useMatches } from '@tanstack/react-router'
 import { SettingsPage } from '@/components/settings/SettingsPage'
 
 export const Route = createFileRoute('/_authenticated/settings')({
-  component: SettingsPage,
+  component: SettingsLayout,
 })
+
+function SettingsLayout() {
+  const matches = useMatches()
+  // If we're on a child route (credentials, snmp-profiles, api-keys), render the Outlet.
+  // If we're on the exact /settings route, render the SettingsPage index.
+  const isChildRoute = matches.some(
+    (m) => m.routeId !== '/_authenticated/settings' && m.routeId.startsWith('/_authenticated/settings/')
+  )
+
+  if (isChildRoute) {
+    return <Outlet />
+  }
+
+  return <SettingsPage />
+}