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/store/auth.ts

@@ -1,6 +1,7 @@
 import { create } from "zustand";
 
 import { api, getToken, setToken } from "@/lib/api";
+import { queryClient } from "@/lib/queryClient";
 import type { User } from "@/lib/types";
 import { useE621 } from "@/store/e621";
 
@@ -24,6 +25,9 @@ export const useAuth = create<AuthState>((set) => ({
     });
     setToken(data.token);
     const user = await api<User>("/api/auth/me/");
+    // Never show the previous account's cached data to this one.
+    queryClient.clear();
+    useE621.getState().clear();
     set({ user, ready: true });
   },
 
@@ -33,6 +37,8 @@ export const useAuth = create<AuthState>((set) => ({
       json: { username, password },
     });
     setToken(data.token);
+    queryClient.clear();
+    useE621.getState().clear();
     set({ user: data.user, ready: true });
   },
 
@@ -43,6 +49,7 @@ export const useAuth = create<AuthState>((set) => ({
       // The token may already be invalid; clearing it locally is enough.
     }
     setToken(null);
+    queryClient.clear();
     useE621.getState().clear();
     set({ user: null, ready: true });
   },