settings menu: replace showExpandDetails prop with a children slot

mara, reviewing PR#3780's showExpandDetails boolean prop: "you cannot
just add it like this - if we have more and more options there in
different places we will keep accumulating cruft in the shared
component." SettingsMenu now exposes a plain children slot instead;
the agent page owns its expand-tool-output row entirely (state,
storage, markup) in its own ExpandDetailsSetting component and passes
it in, so the shared component never learns that setting exists.

frontend/packages/agent/src/components/ExpandDetailsSetting.tsx

@@ -0,0 +1,35 @@
+// Agent-terminal-only settings-menu row: "expand tool output panels".
+// Rendered as a `<SettingsMenu>` child (`@hive/shared/settings-menu.js`)
+// rather than a prop on that shared component — `SettingsMenu` is also
+// used by swarm-ui, which has no terminal to apply this to, and a prior
+// attempt to gate it behind a boolean prop (`showExpandDetails`) on the
+// shared component drew mara's review: "you cannot just add it like
+// this - if we have more and more options there in different places we
+// will keep accumulating cruft in the shared component." This file owns
+// its own state/storage entirely; `SettingsMenu` just renders it as a
+// child, no different from any other consumer's markup.
+import { useState } from 'preact/hooks';
+import { getExpandDetailsPref, setExpandDetailsPref } from '@hive/shared/prefs.js';
+
+export function ExpandDetailsSetting() {
+  // Plain localStorage read, not a hook-managed override like
+  // theme/motion — `getExpandDetailsPref`/`setExpandDetailsPref`
+  // (`@hive/shared/prefs.js`) are the existing shared-storage-key pair
+  // the terminal itself already reads live (no round-trip needed here
+  // beyond re-rendering this row's own checkbox state on toggle).
+  const [expandDetails, setExpandDetailsState] = useState(() => getExpandDetailsPref());
+  return (
+    <label class="settings-menu-row">
+      <span>expand tool output</span>
+      <input
+        type="checkbox"
+        checked={expandDetails}
+        onChange={(e) => {
+          const v = (e.target as HTMLInputElement).checked;
+          setExpandDetailsPref(v);
+          setExpandDetailsState(v);
+        }}
+      />
+    </label>
+  );
+}