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/common.css

@@ -564,7 +564,7 @@ hive-side-panel .md td {
    Inverted-gutter layout: the dashboard keeps `<body>` full-bleed
    and wraps their padded content in a single `.page-content` div that
    carries the horizontal gutter. Full-width chrome (the tab strip, the
-   server-warnings banner, the footer) then spans edge-to-edge for free,
+   footer) then spans edge-to-edge for free,
    with no negative-margin breakout hacks. Only matches on pages that opt
    in by adding the wrapper (index.html); FL0W / ST4TS
    are full-bleed with their own `.<page>-main` padding. */
@@ -573,50 +573,14 @@ hive-side-panel .md td {
 }
 
 /* ─── sticky top region ───────────────────────────────────────────────
-   One sticky container per page holding the warning banner above the
-   page chrome (tab bar / page header). common.js (ensureStickyTop)
-   wraps the page's chrome in this on first render and injects the banner
-   as its first child, so the banner + chrome stack in a single sticky
-   context — pinned together — instead of two separate `top:0` stickies
-   colliding (the banner used to overlay the tab bar). */
+   One sticky container per page holding the page chrome (tab bar / page
+   header). common.js (initStickyTop) wraps the chrome in this on load. */
 .sticky-top {
   position: sticky;
   top: 0;
   z-index: 50;
 }
 
-/* ─── server warnings banner ──────────────────────────────────────────
-   Generic top-of-page strip, injected as the first child of `.sticky-top`
-   by common.js (renderServerWarnings) on every page. One row per warning;
-   colour comes from the per-warning `level` (warn = amber, crit = red).
-   Stickiness lives on the `.sticky-top` wrapper, not here. The bar spans
-   the full width edge-to-edge; the dashboard is full-bleed at the
-   body level (their 1.5em gutter lives on an inner `.page-content`
-   wrapper), so the banner is full-width for free. */
-.server-warnings[hidden] {
-  display: none;
-}
-.server-warn {
-  text-align: center;
-  padding: 0.4em 1em;
-  font-size: 0.85em;
-  font-weight: bold;
-  letter-spacing: 0.02em;
-}
-/* Tint composited over the opaque elevated-surface grey (not transparent):
-   the bar is `position: sticky`, so a see-through background would let
-   page content scroll through behind it. */
-.server-warn-warn {
-  color: var(--amber);
-  background: color-mix(in srgb, var(--amber) 16%, var(--bg-elev));
-  border-bottom: 1px solid var(--amber);
-}
-.server-warn-crit {
-  color: var(--red);
-  background: color-mix(in srgb, var(--red) 18%, var(--bg-elev));
-  border-bottom: 1px solid var(--red);
-}
-
 /* ─── stats table ─────────────────────────────────────────────────
    Shared right-aligned numeric table. Used by /stats.html (busiest
    agents) — moved here from dashboard.css when ST4TS became its own