Fix root layout locale initialization warning

[Copilot]

The root +layout.svelte was triggering a Svelte build warning because locale initialization captured a reactive value outside a closure. This updates the layout bootstrap path to keep locale setup reactive without changing the auth/locale restore flow.

• Root layout bootstrap

  • Replaces direct initialization from a captured prop with a derived locale consumed inside a reactive effect.
  • Keeps restoreLocale(...) and auth.restore() in the mount path, but reads from the same derived source.

• Related cleanup

  • Renames the top-bar create-menu boolean derived value to reflect that it is a predicate, not a collection.
  • Makes dataset search null-safe when description is absent.

let { children, data } = $props();
const initialLocale = $derived(data.initialLocale);

$effect(() => {
  initializeLocale(initialLocale);
});

onMount(() => {
  restoreLocale(initialLocale);
  auth.restore();
});