Clear the query cache when the signed-in account changes

Logging out or switching accounts kept the previous user's React Query
cache (follows, feed, cloud, e621 pages), so the new account briefly
saw the old one's followed tags/pools until each query refetched. The
query client now lives in lib/queryClient.ts and login/register/logout
clear it alongside the e621 credential store.

frontend/src/lib/queryClient.ts

@@ -0,0 +1,12 @@
+import { QueryClient } from "@tanstack/react-query";
+
+export const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      retry: 1,
+      staleTime: 30_000,
+      gcTime: 30 * 60_000,
+      refetchOnWindowFocus: false,
+    },
+  },
+});