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/tabs.js

@@ -11,7 +11,7 @@
 // 4G3NTS (containers) domain lives in `./agents.js`.
 
 import { marked } from "marked";
-import { $, openStream, renderServerWarnings } from "./common.js";
+import { $, openStream, initStickyTop } from "./common.js";
 import { el } from "@hive/shared/dom.js";
 import { bindAsyncForms } from "@hive/shared/forms.js";
 import { createTabStrip } from "@hive/shared/tabs.js";
@@ -50,6 +50,8 @@ import { renderLinksMenu } from "./links-menu.js";
 window.marked = marked;
 
 (() => {
+  initStickyTop();
+
   // ─── async forms ────────────────────────────────────────────────────────
   // Shared `data-async` submit interceptor (in common.js, shared with the other
   // pages' bundles). On success it re-runs refreshState unless the form
@@ -135,7 +137,6 @@ window.marked = marked;
       // synchronous read (e.g. the compose autocomplete pulls agent
       // names from here instead of refetching on every keystroke).
       window.__hyperhive_state = s;
-      renderServerWarnings(s.server_warnings);
       // Hive identity: render the swarm/hive name as a headline at the
       // top of the 4G3NTS pane + update the page title once the
       // server-side display names are known. `hive_name` / `swarm_name`