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/skills/index.tsx

@@ -197,6 +197,7 @@ export function SkillsView({ setStatusbarItemGroup: _setStatusbarItemGroup, ...p
         ) : undefined
       }
       onSearchChange={setQuery}
+      searchHidden={mode === 'skills' ? (skills?.length ?? 0) === 0 : (toolsets?.length ?? 0) === 0}
       searchPlaceholder={mode === 'skills' ? 'Search skills...' : 'Search toolsets...'}
       searchValue={query}
       tabs={