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.js

@@ -562,22 +562,12 @@ export function appendLinkified(parent, text, refs) {
   }
 }
 
-// ─── server warnings banner ──────────────────────────────────────────
-// A generic top-of-page banner shown on every page (dashboard + the
-// stand-alone FL0W / ST4TS pages). The backend decides what to
-// warn about — `/api/state.server_warnings` is a list of
-// `{ kind, level, message }` — and this just renders it, coloured by
-// `level` (`warn` amber / `crit` red). Adding a new system warning is a
-// backend-only change. The bar is injected at the top of <body> so no
-// page needs to add markup.
-
-// The sticky top region — one sticky container holding the warning
-// banner above the page's chrome (tab bar / page header), so the banner
-// stacks with the chrome instead of being overlaid by it (two separate
-// `top:0` stickies would otherwise collide). Built once by wrapping the
-// page's existing chrome element; pages without a chrome (none today)
-// get a banner-only sticky region at the top of <body>.
-function ensureStickyTop() {
+// ─── sticky top region ───────────────────────────────────────────────
+// One sticky container wrapping the page's chrome (tab bar / page
+// header), built once by wrapping the existing chrome element. A page
+// without a chrome (none today) gets an empty sticky region at the top of
+// <body>.
+export function initStickyTop() {
   let top = document.querySelector(".sticky-top");
   if (top) return top;
   top = document.createElement("div");
@@ -591,51 +581,3 @@ function ensureStickyTop() {
   }
   return top;
 }
-
-function ensureServerWarningsBar() {
-  let bar = document.getElementById("server-warnings");
-  if (!bar) {
-    bar = document.createElement("div");
-    bar.id = "server-warnings";
-    bar.className = "server-warnings";
-    bar.setAttribute("role", "alert");
-    bar.hidden = true;
-    ensureStickyTop().prepend(bar);
-  }
-  return bar;
-}
-
-/// Render a `server_warnings` list (from /api/state) into the banner.
-/// Empty / missing → the bar hides itself.
-export function renderServerWarnings(warnings) {
-  const bar = ensureServerWarningsBar();
-  bar.replaceChildren();
-  if (!Array.isArray(warnings) || warnings.length === 0) {
-    bar.hidden = true;
-    return;
-  }
-  for (const w of warnings) {
-    const row = el("div", {
-      class:
-        "server-warn server-warn-" +
-        (w && w.level === "crit" ? "crit" : "warn"),
-    });
-    appendText(row, "⚠ " + ((w && w.message) || ""));
-    bar.append(row);
-  }
-  bar.hidden = false;
-}
-
-/// One-shot init for pages that don't otherwise poll /api/state: ensure
-/// the bar exists, fetch the snapshot once, render. The dashboard (which
-/// already polls /api/state) calls `renderServerWarnings` directly for
-/// live updates instead.
-export function initServerWarnings() {
-  ensureServerWarningsBar();
-  fetch("/api/state")
-    .then((r) => (r.ok ? r.json() : null))
-    .then((s) => renderServerWarnings(s && s.server_warnings))
-    .catch(() => {
-      /* non-fatal: no banner if the snapshot is unreachable */
-    });
-}