agent: meta-nav, favicon fallback, document.title

Ports the 3 remaining gaps argus's lost-functionality audit found beyond
login: the header meta-nav strip (stats/screen/forge/config +
hyperhive.dashboardLinks extras — the only on-page path to those, not
just polish), the header icon's /favicon.svg fallback on a broken
image, and the browser tab title update. All three are straight ports
of app.js's existing logic (refreshState's meta-links loop,
bindHeaderIconFallback, setHeader's document.title block), same
kind -> URL resolution rules, same dataset.fallback loop guard, same
qualified_label/hive_name fallback chain — no new backend fields
needed, hive_agent::web_ui::state::StateSnapshot already serves
links/forge_public_url and useAgentState's whole-payload cast already
threads them to Root.

New <MetaNav> component (reuses agent.css's existing .agent-nav/
.agent-nav-link rules verbatim, no new CSS) renders in Header's title
row via a new nav prop. Favicon fallback lives in Header itself
(onError handler on the .agent-icon <img>). document.title is a
useEffect in Root keyed on label/qualified_label/hive_name.

tsc --noEmit clean, build clean, both pre-push lints clean. Screenshot
verifies meta-nav renders 4 links (stats/forge/config/extra, forge
kind resolving against a mocked forge_public_url) and the favicon
fallback firing against a deliberately-404'd /icon.

frontend/packages/agent/src/components/MetaNav.tsx

@@ -0,0 +1,44 @@
+// <MetaNav> — the header's meta-nav strip (`<nav id="meta-links">` in
+// the old markup): stats/screen/forge/config + any
+// `hyperhive.dashboardLinks` extras, sourced from the backend's
+// `agent_links()` (the single source of truth — same list also feeds
+// the dashboard card's icon strip). See
+// docs/web-ui/agent.md::Header for the kind → URL resolution table
+// this ports verbatim from app.js's `refreshState` (lines ~1216-1244):
+// `container` → same-origin path (this page is itself container-local);
+// `forge` → `forgePublicUrl + url`, and the link is dropped entirely
+// when `forgePublicUrl` is unset (never guessed from `<host>:3000`);
+// `external` → already absolute. Plain JSX text (not `innerHTML`), so
+// this is XSS-safe by construction the same way app.js's `el()`-built
+// anchors were — no new escaping to get right.
+import type { AgentLink } from '../types.js';
+
+export interface MetaNavProps {
+  links: AgentLink[];
+  forgePublicUrl: string | null;
+}
+
+export function MetaNav({ links, forgePublicUrl }: MetaNavProps) {
+  const visible = links.filter((lnk) => lnk.kind !== 'forge' || forgePublicUrl);
+  if (visible.length === 0) return null;
+
+  return (
+    <nav class="agent-nav" id="meta-links">
+      {visible.map((lnk) => {
+        const href = lnk.kind === 'forge' ? `${forgePublicUrl}${lnk.url}` : lnk.url;
+        return (
+          <a
+            key={lnk.url}
+            class="agent-nav-link"
+            href={href}
+            target="_blank"
+            rel="noopener"
+            title={lnk.label}
+          >
+            {`${lnk.icon} ${lnk.label}`.trim()} →
+          </a>
+        );
+      })}
+    </nav>
+  );
+}