swarm-ui: serve the hyperhive favicon

Same pattern frontend.nix already uses for the dashboard: the SVG
lives outside the npm tree at branding/hyperhive.svg, copied into
$out during the nix install phase (not build.mjs, so plain npm build
consumers still work, same split frontend.nix uses). Added the
matching <link rel="icon"> to index.html.

nix/packages/default.nix

@@ -188,7 +188,9 @@ in
   # above, kept a separate package for the same reason: not every hive
   # runs the swarm controller, so this shouldn't ride along in
   # `packages.default`'s closure.
-  swarm-ui = pkgs.callPackage ./swarm-ui.nix { };
+  swarm-ui = pkgs.callPackage ./swarm-ui.nix {
+    branding-svg = ../../branding/hyperhive.svg;
+  };
 
   # Bundled browser assets — see ./frontend.nix. Output is
   # $out/{dashboard,agent}/ which the Rust binaries serve via