swarm-ui: show the swarm's name as the page title and top-left brand

New GET /api/swarm on swarm-controller, backed by
services.hyperhive.swarm.name (SWARM_CONTROLLER_NAME env var, same
optionalAttrs-gated-on-option-resolving shape queueEnv/forgeEnv/etc.
already use). swarm-ui's <Shell> fetches it once and sets both
document.title and the header's brand text; falls back to the
existing static "hyperhive swarm" label when the operator never set
a name or the fetch fails.

Extracted the swarm-queue connect block out of main() into its own
connect_status_reader() fn to keep main() under clippy's line-count
lint after adding the new field wiring — no behavior change, same
comments moved as-is.

nix/host-modules/swarm-controller.nix

@@ -141,6 +141,14 @@ let
     SWARM_CONTROLLER_AUTH_BRIDGE_URL = cfg.authBridgeUrl;
   };
 
+  # The swarm's own display name, for `GET /api/swarm` (swarm-ui's chrome).
+  # Gated on the option resolving, not defaulted to an empty string: an
+  # operator who never named the swarm gets `name: null` from the
+  # endpoint, not a blank label rendered as if it meant something.
+  swarmNameEnv = lib.optionalAttrs (config.services.hyperhive.swarm.name != null) {
+    SWARM_CONTROLLER_NAME = config.services.hyperhive.swarm.name;
+  };
+
   # Wrapped rather than documented: every one of these values is derived
   # from an option this deployment already set, so making the operator
   # re-supply them on the command line would be asking them to repeat the
@@ -653,7 +661,8 @@ in
       // queueEnv
       // forgeEnv
       // webhookEnv
-      // authBridgeEnv;
+      // authBridgeEnv
+      // swarmNameEnv;
     };
 
     # A systemd credential is a SNAPSHOT: it is materialised into `%d` once,