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.

hive-c0re/src/workers/crash_watch.rs

@@ -149,7 +149,7 @@ fn emit_crash_transitions(coord: &Coordinator, prev: &HashSet<String>, current:
         // The one place this fires: a swarm-level grafana alert
         // (`hh-container-crashed` in `nix/host-modules/swarm-grafana.nix`)
         // matches this exact message against the log store and pages the
-        // operator directly — no agent (and no per-hive dashboard banner)
+        // operator directly — no agent (and no per-hive warning)
         // in the loop. See `docs/scheduler/observability.md`.
         tracing::warn!(agent = %stopped, "container crash detected");
     }