swarm-ui: a start page at / and the hive directory at /hives

/ is now the swarm's name over the digital-rain backdrop the hive
dashboard's home has, and the swarm name in the header links to it. The
hive directory moves to /hives, where the nav's hives tab points. The
swarm name is read once per page load by a shared hook, the header and
the start page both use it. Closes #5252

frontend/packages/swarm-ui/src/pages/StartPage.tsx

@@ -0,0 +1,20 @@
+// <StartPage> — `/`, the swarm's landing page: the swarm's name over the
+// digital-rain backdrop, and nothing else. The header's swarm name links
+// here. The hive roster that used to be `/` is now `/hives`.
+import { MatrixRain } from "../shell/MatrixRain.js";
+import { useSwarmName } from "../shell/useSwarmName.js";
+import "./StartPage.css";
+
+// Same fallback as the header and the page title, so an unnamed swarm shows
+// one label everywhere.
+const DEFAULT_NAME = "hyperhive swarm";
+
+export function StartPage() {
+  const name = useSwarmName() ?? DEFAULT_NAME;
+  return (
+    <div class="start-page">
+      <MatrixRain />
+      <h1 class="start-page-name">{name}</h1>
+    </div>
+  );
+}