Show which cluster an incident came from, as a chip

A team with one Alertmanager per Kubernetes cluster could not tell at a
glance where an incident started: the cluster was only a word inside the
title. When an alert carries a `cluster` label, the queue rows, the incident
page and the alerts list now show it as a chip in that cluster's colour, a
stable pick from the existing six-colour palette. A queue row drops
`cluster=...` from its title, since the chip says it, and the incident page
keeps the full title.

An incident has the label only when it is in Alertmanager's group_by, which
is also what keeps two clusters' identical alerts apart: incidents are matched
on the team and the groupKey, and the groupKey does not include external
labels. The README has a section on the two settings (Prometheus
externalLabels and group_by). The alerts list reads the label from the alert
itself, so it shows the chip with only the external label.

Web UI and docs only: no endpoint or JSON shape changed, so nothing to mirror
in terdut-tui. Nothing changes for a team whose alerts have no cluster label.

internal/web/static/js/format.js

@@ -117,3 +117,36 @@ export function labelSummary(labels, skip = 'alertname') {
 export function initial(name) {
   return (name || '?').trim().charAt(0) || '?';
 }
+
+// Where an incident or alert came from, for a team with several Kubernetes
+// clusters (or other origins) behind it: the value of one label, `cluster` by
+// convention. It comes from Prometheus's externalLabels, so it is on every
+// alert; an incident carries it only when it is in Alertmanager's group_by,
+// which is also what keeps two clusters' identical alerts from merging into one
+// incident (see the README, "Several clusters, one team").
+export const ORIGIN_LABEL = 'cluster';
+
+export function originOf(labels) {
+  const v = labels && labels[ORIGIN_LABEL];
+  return v ? String(v) : null;
+}
+
+// A stable colour for an origin, from the six-colour rcN palette app.css has,
+// so "prod-eu" is the same colour on every row without anything being stored.
+export function originClass(value) {
+  let n = 0;
+  for (const ch of value) n = (n * 31 + ch.charCodeAt(0)) >>> 0;
+  return `rc${(n % 6) + 1}`;
+}
+
+// The title without its `cluster=...` part, for a row that shows the origin as
+// a chip: the server puts every grouping label in the title, so the cluster
+// would otherwise appear twice. Anything that does not look like the server's
+// "name (k=v, k=v)" shape is returned unchanged.
+export function titleWithoutOrigin(title, value) {
+  if (!value) return title;
+  const m = title.match(/^(.*?) \((.*)\)$/s);
+  if (!m) return title;
+  const rest = m[2].split(', ').filter((p) => p !== `${ORIGIN_LABEL}=${value}`);
+  return rest.length ? `${m[1]} (${rest.join(', ')})` : m[1];
+}