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.

docs/swarm/README.md

@@ -148,8 +148,8 @@ swarm-services certificate.
 The identity isn't a choice — a hive authenticates as `hive-<hiveName>`
 and publishes under `hiveName`, the same name that keys `swarm.hives`.
 
-If a hive stops reporting, its own dashboard is the place to look: a
-failure to publish raises a warning banner there after three consecutive
+If a hive stops reporting, its `/health/ready` and the `hyperhive.hive.warning` metric are the place to look: a
+failure to publish raises a hive warning after three consecutive
 misses. It stays `warn` rather than `crit` on purpose — a hive that
 can't reach the queue isn't itself unhealthy, so it doesn't start
 calling itself degraded for being unable to say it's fine.