swarm-ui: test the rain arithmetic and the swarm name loader

The rain's column maths and fade colour move from MatrixRain into
shell/rain.ts, and the request-once swarm name loader from useSwarmName
into shell/swarm-name.ts. Both are plain functions with their input
passed in, so tests cover them; the components keep the DOM and canvas
work.

frontend/packages/swarm-ui/src/shell/swarm-name.ts

@@ -0,0 +1,28 @@
+// Loading the swarm's display name from `GET /api/swarm`, once. The header and
+// the start page both ask for it and it does not change within a page visit,
+// so the loader remembers the first request and every caller shares it. The
+// label is cosmetic, so a failed request, an error response or an unconfigured
+// name all answer `null` instead of throwing: callers fall back to their own
+// default, as an operator who never set a name sees.
+
+type Fetch = (url: string) => Promise<{
+  ok: boolean;
+  json: () => Promise<unknown>;
+}>;
+
+export function createSwarmNameLoader(
+  fetchImpl: Fetch,
+): () => Promise<string | null> {
+  let pending: Promise<string | null> | null = null;
+  return () => {
+    if (pending === null) {
+      pending = (async () => {
+        const r = await fetchImpl("/api/swarm");
+        if (!r.ok) return null;
+        const data = (await r.json()) as { name?: string | null };
+        return data.name || null;
+      })().catch(() => null);
+    }
+    return pending;
+  };
+}