feat(alerts): pick the host, then its part, and see what the metric reads (#5475)

#5475 "Rule form: pick the host, then its part; metrics that part reports;
a link to groups; a preview of the metric", from the operator's review of
the New rule page: hosts and their parts shared one long picker.

- Host agent rules pick a Host, then a Part: "The whole host", then its
  mounts, CPU cores, disks, interfaces and sensors under a heading each,
  cores in number order. A part that reports nothing a rule can watch
  (an interface's traffic) isn't offered.
- The Metric list holds what the picked resource reports, in catalog order.
- Under it, a preview: the reading now, and the last 24 h low to high with
  an hourly sparkline drawn from zero (UI ruling 8), read from one series.
- The group picker links to Ansible > Inventory to make a group, in a new
  tab, and re-reads the groups when the tab comes back into view.
- Picking a host or part swaps the pickers alone (/alerts/api/rule_target);
  picking a metric swaps the preview (/alerts/api/metric_preview).
- A rule's own resource and metric stay on offer after they go quiet.

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

steward/static/steward.css

@@ -219,6 +219,11 @@ strong, b, th { font-weight: 500; }
 /* A covering alert rule's per-host rules (#5464), indented under it. */
 .table tr.row-child > td { border-top-style: dashed; }
 .table tr.row-child .cell-name { padding-left: 2rem; }
+/* The rule form's metric preview (#5475): now, then the last day beside its sparkline. */
+.metric-preview { display: flex; flex-wrap: wrap; align-items: center; gap: 0.25rem 1rem;
+  margin-top: 0.4rem; font-size: 0.85rem; }
+.metric-preview-day { display: inline-flex; align-items: center; gap: 0.4rem; }
+.metric-preview-spark { color: var(--text-muted); line-height: 0; }
 /* A row that opens as a whole: its one .row-target link stretches over the
    row, and the actions cell sits above it so its buttons keep their own
    clicks. One real link per row, so keyboard and screen readers meet it once. */