e621 credentials: user fields, API endpoint, Account screen

Backend:
- User model gains e621_username / e621_api_key / e621_base_url
- GET/PUT /api/auth/e621/ for the owner's credentials; /me exposes only
  the username and a configured flag, never the key

Frontend:
- e621 client core: Basic auth, _client param (browsers cannot set a
  User-Agent), serialized queue throttled to 1 request/second, readable
  error mapping
- Account screen (/account): username, API key with reveal toggle,
  base URL (e621 / e926 / custom), Save + Test connection
- Credentials are fetched from the backend and held in memory only,
  cleared on logout

frontend/src/store/auth.ts

@@ -2,6 +2,7 @@ import { create } from "zustand";
 
 import { api, getToken, setToken } from "@/lib/api";
 import type { User } from "@/lib/types";
+import { useE621 } from "@/store/e621";
 
 interface AuthState {
   user: User | null;
@@ -42,6 +43,7 @@ export const useAuth = create<AuthState>((set) => ({
       // The token may already be invalid; clearing it locally is enough.
     }
     setToken(null);
+    useE621.getState().clear();
     set({ user: null, ready: true });
   },