web: sectioned /settings with saved-searches CRUD

Add four TanStack Query hooks (useSavedSearches, useCreateSavedSearch,
useUpdateSavedSearch, useDeleteSavedSearch) backed by IF3 contract.
Introduce apiFetchVoid in client.ts for the 204 No Content DELETE path.
Replace the placeholder /settings route with a two-column sectioned shell
(SectionRail + SavedSearchesSection); Display section is disabled pending #8.