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/swarm_notices.rs

@@ -14,7 +14,7 @@
 //! coordinates is a silent no-op (the ordinary case). A hive that cannot
 //! reach the queue loses the swarm's visibility of the notice, not the
 //! host's — `warn!` fires on every failed attempt regardless, and the
-//! dashboard banners only after [`FAILURES_BEFORE_BANNER`] consecutive
+//! raises a hive warning only after [`FAILURES_BEFORE_WARNING`] consecutive
 //! misses, the same debounce shape [`crate::swarm_status`] uses and for
 //! the same reason: a `warn!` that fires every call for three weeks is
 //! indistinguishable from silence in practice.
@@ -24,10 +24,10 @@ use tokio::sync::{Mutex, OnceCell};
 
 use crate::stats::sweep_health::{self, SweepHealth};
 
-/// Consecutive failed publishes before the dashboard banners — same
+/// Consecutive failed publishes before the sweep raises a hive warning — same
 /// value [`crate::swarm_status`] uses and for the same reason: a debounce
-/// against one blip flapping a banner an operator learns to ignore.
-const FAILURES_BEFORE_BANNER: u32 = 3;
+/// against one blip flapping a warning an operator learns to ignore.
+const FAILURES_BEFORE_WARNING: u32 = 3;
 
 static HEALTH: OnceCell<Mutex<SweepHealth>> = OnceCell::const_new();
 
@@ -37,7 +37,7 @@ async fn health() -> tokio::sync::MutexGuard<'static, SweepHealth> {
             Mutex::new(SweepHealth::new(
                 "swarm_notices_publish",
                 "warn",
-                FAILURES_BEFORE_BANNER,
+                FAILURES_BEFORE_WARNING,
             ))
         })
         .await
@@ -57,7 +57,7 @@ pub async fn notify(subsystem: &str, key: Option<String>, summary: String, sourc
         return;
     };
     // Same absent-name condition `swarm_status` bails on — that module
-    // already banners it under `swarm_status_config` the first time
+    // already warns it under `swarm_status_config` the first time
     // either of us hits it; nothing more to add from here.
     let Some(hive) = crate::container_view::hive_swarm_names().0 else {
         return;