Ask for JSON when autocompleting people to ping

`as: "json"` is a @rails/request.js option; plain fetch ignores it and
sends `Accept: */*`, so Autocompletable::UsersController answered with
its HTML format and response.json() threw. The new ping form never
showed any suggestions. Send an explicit Accept header instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/javascript/lib/autocomplete/base_autocomplete_handler.js

@@ -120,7 +120,7 @@ export default class BaseAutocompleteHandler {
 
   #fetchAutocompletables(url) {
     if (url) {
-      return fetch(url, { as: "json" }).then(response => response.json())
+      return fetch(url, { headers: { Accept: "application/json" } }).then(response => response.json())
     } else {
       return Promise.resolve()
     }