swarm-ui: the jobs and builds cards are folded by default

Both cards on the jobs tab start shut and read nothing until opened; a link
with ?run= opens the builds card. The fold-and-build-on-first-open part of the
hives card is now one LazyDisclosure, which the hives card uses too. Docs
follow.

frontend/packages/swarm-ui/src/ui/disclosure/LazyDisclosure.tsx

@@ -0,0 +1,40 @@
+// <LazyDisclosure> — a Disclosure that is shut by default and builds its body
+// on the first open. For a card that polls or fetches: one nobody opens costs
+// nothing, and one that was opened keeps its body, and what it read, after a
+// fold. `startOpen` opens it from the first render, for a link that points
+// into it.
+import { useState } from "preact/hooks";
+import type { ComponentChildren } from "preact";
+import { Disclosure } from "./Disclosure.js";
+import { builtAfterToggle } from "./lazy-open.js";
+
+export function LazyDisclosure({
+  title,
+  icon,
+  hint,
+  startOpen = false,
+  children,
+}: {
+  title: string;
+  icon?: string;
+  hint?: ComponentChildren;
+  startOpen?: boolean;
+  children: ComponentChildren;
+}) {
+  const [open, setOpen] = useState(startOpen);
+  const [built, setBuilt] = useState(startOpen);
+  return (
+    <Disclosure
+      title={title}
+      icon={icon}
+      hint={hint}
+      open={open}
+      onToggle={(next) => {
+        setOpen(next);
+        setBuilt((was) => builtAfterToggle(was, next));
+      }}
+    >
+      {built ? children : null}
+    </Disclosure>
+  );
+}

frontend/packages/swarm-ui/src/ui/disclosure/lazy-open.ts

@@ -0,0 +1,5 @@
+// A folded card builds its body on the first open and keeps it after, so a
+// card nobody opens never fetches, and closing it does not drop what it read.
+export function builtAfterToggle(built: boolean, open: boolean): boolean {
+  return built || open;
+}