polish(web): search pill aria-pressed, keepPreviousData, plural result count, URL-hydration test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/api/queries.ts

@@ -1,4 +1,4 @@
-import { useInfiniteQuery, useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { keepPreviousData, useInfiniteQuery, useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 
 import { api } from "./client";
 import type { components } from "./schema";
@@ -306,6 +306,7 @@ export function useSearch(q: string, visibility: string | null) {
 
       return data;
     },
+    placeholderData: keepPreviousData,
     getNextPageParam: (lastPage, allPages) => {
       const loaded = allPages.reduce((n, page) => n + page.hits.length, 0);