hive-c0re: hive warnings reach the operator as metrics and alerts, not a banner

The dashboard's top-of-page warning banner is gone. The warning registry
stays, because /health/ready, the swarm status view and the OTEL gauges
read it. What it feeds is now alerted on in Grafana:

- hyperhive.hive.warning{kind,level}: one series per raised warning, so an
  alert names the cause. Raising a warning also logs one `hive warning
  raised` line with the message.
- hyperhive.hive.nix_store.utilization: the used fraction of the nix
  store's filesystem, probed in the gauge callback.
- Alerts: hh-hive-degraded on the existing degraded gauge, hh-hive-warning
  per kind (disk excluded), hh-host-disk-high at 85% and
  hh-host-disk-critical at 95%.
- pending_logins is now a registry warning, refreshed from the container
  rescan, so it reaches the per-kind gauge.

Frontend: server_warnings leaves the state snapshot, and the dashboard,
flow and stats pages keep only the sticky chrome wrapper. The shared
hive-warn and WarnBanner components stay, since other views use them.

A module-eval case checks each alert's metric name against the exporter's
source, so a rename on either side fails the build.

frontend/packages/shared/src/chrome.css

@@ -16,8 +16,7 @@
 */
 .page-header {
   /* Stickiness lives on the `.sticky-top` wrapper (common.css) that
-     common.js wraps this header in, alongside the server-warning banner,
-     so the two stack instead of colliding. */
+     common.js wraps this header in. */
   background: color-mix(in srgb, var(--bg) 92%, transparent);
   -webkit-backdrop-filter: blur(8px) saturate(120%);
   backdrop-filter: blur(8px) saturate(120%);