fix(web): tidy channel health policy layout

Lay out the channel health form with the shared settings grid so switch
rows span the full width and inputs align in two columns instead of a
fixed three-column grid that stretched the switches. Nest the test mode
and interval under the scheduled tests switch, make the mode select fill
its column, and replace the alert boxes and channels link at the top with
plain hint text. Add a layout regression test and the new group label
translation.