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/flow.js

@@ -10,17 +10,12 @@
 // container list).
 
 import { create as termCreate } from "@hive/shared/terminal.js";
-import {
-  $,
-  appendLinkified,
-  openStream,
-  initServerWarnings,
-} from "./common.js";
+import { $, appendLinkified, openStream, initStickyTop } from "./common.js";
 import { el } from "@hive/shared/dom.js";
 import { epochSec } from "./util.js";
 
 (() => {
-  initServerWarnings();
+  initStickyTop();
 
   // ─── local containers cache (for compose autocomplete) ──────────────────
   // The compose box's @-mention completion suggests known agent names.