style(desktop): hide search when there's nothing to search

Empty datasets no longer render a search field. Adds a `searchHidden` prop
to PageSearchShell (artifacts/skills/messaging) and gates cron + command
center sessions search on a non-empty list. The chat sidebar already did
this via showSessionSections.

apps/desktop/src/app/artifacts/index.tsx

@@ -501,6 +501,7 @@ export function ArtifactsView({ setStatusbarItemGroup: _setStatusbarItemGroup, .
     <PageSearchShell
       {...props}
       onSearchChange={setQuery}
+      searchHidden={counts.all === 0}
       searchPlaceholder="Search artifacts..."
       searchValue={query}
       tabs={