P2R-02 slice 2 follow-up: refuse to delete a host's last source group

Belt-and-braces: the UI now disables the Delete button when a group
is the only one on the host (with a tooltip explaining why), and the
server-side handler returns 409 if a curl/form-replay tries anyway.
Every host needs at least one source group to be backup-able, so the
'last group on a fresh host' case is a meaningful accident to guard
against.

web/templates/pages/host_sources.html

@@ -66,6 +66,9 @@
             {{if gt $row.UsedBy 0}}
               <button class="btn btn-danger" disabled
                       title="remove this group from {{$row.UsedBy}} schedule{{if ne $row.UsedBy 1}}s{{end}} first">Delete</button>
+            {{else if eq (len $page.Groups) 1}}
+              <button class="btn btn-danger" disabled
+                      title="this is the host's only source group — create another one first">Delete</button>
             {{else}}
               <form method="post" action="/hosts/{{$host.ID}}/sources/{{$g.ID}}/delete" style="display: inline;"
                     onsubmit="return confirm('Delete source group &quot;{{$g.Name}}&quot;? Existing snapshots are not affected.');">