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/swarm-ui.nix

@@ -1,4 +1,4 @@
-{ buildNpmPackage }:
+{ buildNpmPackage, branding-svg }:
 
 # Static build of the swarm-level UI shell (project-bootstrap scope:
 # empty start page, no functionality yet — see
@@ -27,7 +27,9 @@
 # `npm run build --workspaces` frontend.nix runs) via an explicit
 # `npm run build --workspace=…` buildPhase, so a dashboard/agent-only
 # change doesn't need to rebuild (or re-review the output of) this
-# derivation.
+# derivation. The favicon (`branding/hyperhive.svg`, same file
+# `./frontend.nix` copies for the dashboard) is likewise copied in
+# during install — it lives outside the npm tree.
 
 buildNpmPackage {
   pname = "hyperhive-swarm-ui";
@@ -48,6 +50,7 @@ buildNpmPackage {
     runHook preInstall
     mkdir -p $out
     cp -r packages/swarm-ui/dist/. $out/
+    cp ${branding-svg} $out/favicon.svg
     runHook postInstall
   '';