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/swarm_limits.rs

@@ -24,11 +24,11 @@ use crate::stats::sweep_health::{self, SweepHealth};
 pub const READ_INTERVAL: Duration = Duration::from_secs(30);
 
 /// At [`READ_INTERVAL`] this is ~90 seconds of failing reads.
-const FAILURES_BEFORE_BANNER: u32 = 3;
+const FAILURES_BEFORE_WARNING: u32 = 3;
 
 /// Read the caps now and every [`READ_INTERVAL`] until shutdown.
 pub async fn follow(client: async_nats::Client, mut shutdown: tokio::sync::watch::Receiver<bool>) {
-    let mut health = SweepHealth::new("swarm_limits_read", "warn", FAILURES_BEFORE_BANNER);
+    let mut health = SweepHealth::new("swarm_limits_read", "warn", FAILURES_BEFORE_WARNING);
     loop {
         match sync(&client).await {
             Ok(()) => health.record_ok(),