dashboarddefaults: pick a kiosk from a dropdown (auto-fill FQDN) + Location wording

Adds GET /api/computers/display-kiosks - the displays that reported in (Kiosk
type), each with its derived FQDN (F<serial>.<domain>). The Dashboard Defaults
form gets a kiosk dropdown that fills the FQDN so admins pick a display instead
of typing an IP; IP stays an optional manual field. Table shows FQDN or IP.
'Business Unit' label -> 'Location' on this page + the settings nav.

frontend/src/api/index.js

@@ -120,6 +120,9 @@ export const computersApi = {
   list(params = {}) {
     return api.get('/computers', { params })
   },
+  displayKiosks() {
+    return api.get('/computers/display-kiosks')
+  },
   get(id) {
     return api.get(`/computers/${id}`)
   },