dashboard + agent UI: surface hive_name + swarm_name in chrome (#701)

Damocles's backend (c41bf1b) landed HYPERHIVE_HIVE_NAME / _SWARM_NAME
env vars and identity.rs accessors. This commit wires them into the two
web surfaces:

hive-c0re/dashboard.rs:
  StateSnapshot gains `hive_name` + `swarm_name` (Option<String>),
  populated from the env vars the c0re NixOS module injects.

hive-ag3nt/web_ui.rs:
  Per-agent StateSnapshot gains the same two fields, populated from
  crate::identity::hive_name() / swarm_name().

Dashboard frontend (index.html + tabs.js):
  A `<p class="banner-thin" id="hive-identity">` sits above the tab
  strip in the chrome. refreshState() populates it with "swarm / hive"
  (or just "hive") when the fields are non-null, and updates
  document.title to "<swarm>/<hive> // h1ve-c0re". No change to
  chrome when both fields are null (backward compat).

Per-agent frontend (index.html + app.js + agent.css):
  A hidden `.agent-hive-label` span under the title row shows the
  swarm/hive label once setHeader() receives non-null names.
  document.title is updated to "<label> // <hive_name>" when hive_name
  is set, enabling browser-tab disambiguation when multiple hives are
  open in parallel. Styled as a small uppercase purple-dim sub-label.

frontend/packages/dashboard/src/index.html

@@ -14,6 +14,10 @@
        button / shared links keep you on the same view. JS owns
        the show/hide. -->
   <header class="dashboard-chrome">
+    <!-- Hive identity line: swarm/hive label. Hidden until JS populates
+         it from /api/state (hive_name + swarm_name). Falls back to
+         hostname when only the domain is configured. -->
+    <p class="banner-thin" id="hive-identity" hidden></p>
     <nav class="tabbar" id="tabbar" role="tablist">
       <a class="tab" id="tab-swarm" href="#swarm" role="tab"
          aria-controls="tab-pane-swarm"