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/forge/repos.rs

@@ -125,7 +125,7 @@ pub async fn push_meta(dir: &Path) -> Result<()> {
             let msg = "forge/core/meta push rejected (non-fast-forward): \
                        remote is ahead of local; leaving remote history intact";
             tracing::warn!("{msg}");
-            // Raise a persistent dashboard banner; cleared on next successful push.
+            // Raise a persistent hive warning; cleared on next successful push.
             if let Ok(mut g) = META_NON_FF_GUARD.lock() {
                 *g = Some(crate::warnings::set_warning(
                     "forge-meta-non-ff",