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/stats.js

@@ -5,7 +5,7 @@
 // this bundle has no chart lib; per-agent trend charts live on each
 // agent's own /stats page. The window selector is a hash-routed
 // createTabStrip (#1h / #24h / …), matching the per-agent stats page.
-import { $, initServerWarnings } from "./common.js";
+import { $, initStickyTop } from "./common.js";
 import { createTabStrip } from "@hive/shared/tabs.js";
 
 let hiveStatsWindow = "24h";
@@ -173,7 +173,7 @@ async function refreshHiveStats() {
   }
 }
 
-initServerWarnings();
+initStickyTop();
 
 // Window selector → hash-routed createTabStrip (#1h / #24h / …). No panes
 // here (the selector swaps data, not DOM); paneFor() returns null for each