WHY: htmx sends non-GET requests urlencoded by default; the SetFeedOrder
request struct only carried a json tag, so the binder left Order empty
and 'required' fired — the exact error the order dropdown AND the
Serial-suggestion button produced (hx-vals JSON also arrives
form-encoded). One form tag fixes both controls.
Regression test drives the exact wire format: urlencoded PUT with
order=oldest/native → 200, empty/invalid → 400.