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/ui.js

@@ -1,6 +1,6 @@
 // DOM helpers, the bottom sheet, confirmation and toasts.
 
-import { severityClass } from './format.js';
+import { severityClass, originClass } from './format.js';
 
 // h builds an element. attrs: class, text, on<event>, dataset, aria/other
 // attributes; boolean true sets an empty attribute, false/null skips it.
@@ -49,6 +49,7 @@ export function fadeOnOverflow(el) {
 
 // Stroke icons, 24×24. Built as SVG nodes so the CSP needs no inline anything.
 const ICONS = {
+  server: ['rect:3,4,18,6,2', 'rect:3,14,18,6,2', 'M7 7h.01', 'M7 17h.01'],
   alertTriangle: ['M12 4l9.5 16.5h-19z', 'M12 10v4.5', 'M12 17.5h.01'],
   alertCircle: ['circle:12,12,9', 'M12 7.5v5', 'M12 16h.01'],
   infoCircle: ['circle:12,12,9', 'M12 11v5.5', 'M12 7.5h.01'],
@@ -193,6 +194,13 @@ export function badge(text, cls = '') {
   return h('span', { class: `badge ${cls}`, text });
 }
 
+// Where it came from (the cluster): a chip in that origin's colour, with a
+// server icon so it reads as a place and not as a status.
+export function originChip(value) {
+  return h('span', { class: `origin-chip ${originClass(value)}`, title: `Origin: ${value}` },
+    icon('server', 'icon badge-icon'), value);
+}
+
 // The severity as a badge with a shape as well as a colour: a triangle for
 // critical, a circle with ! for warning, a circle with i for anything else, so
 // the coloured bar and badge are not the only thing that says it.