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.

frontend/packages/dashboard/src/dashboard.css

@@ -17,7 +17,7 @@
 body.dashboard-shell {
   /* Full-bleed body (no horizontal gutter). The 1.5em side gutter lives
      on the inner `.page-content` wrapper (see common.css), so the sticky
-     chrome bar, the server-warnings banner, and the footer span the full
+     chrome bar and the footer span the full
      width edge-to-edge with no negative-margin breakout. `padding-bottom`
      keeps a little breathing room above the viewport edge (overridden to
      clear the selection bar via `.has-selection` below). No max-width cap
@@ -34,8 +34,7 @@ body.dashboard-shell {
   align-items: center;
   gap: 0.75em;
   /* Stickiness lives on the `.sticky-top` wrapper (common.css) that
-     common.js wraps this chrome in, alongside the server-warning banner,
-     so they stack in one sticky context instead of colliding. */
+     common.js wraps this chrome in. */
   background: color-mix(in srgb, var(--bg) 86%, transparent);
   -webkit-backdrop-filter: blur(8px) saturate(120%);
   backdrop-filter: blur(8px) saturate(120%);