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/main.tsx

@@ -1,21 +1,11 @@
-import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { QueryClientProvider } from "@tanstack/react-query";
 import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import { BrowserRouter } from "react-router-dom";
 
 import App from "./App";
 import "./index.css";
-
-const queryClient = new QueryClient({
-  defaultOptions: {
-    queries: {
-      retry: 1,
-      staleTime: 30_000,
-      gcTime: 30 * 60_000,
-      refetchOnWindowFocus: false,
-    },
-  },
-});
+import { queryClient } from "@/lib/queryClient";
 
 createRoot(document.getElementById("root")!).render(
   <StrictMode>