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 });
   },
 

frontend/src/store/e621.ts

@@ -0,0 +1,42 @@
+import { create } from "zustand";
+
+import { api } from "@/lib/api";
+import type { E621Credentials } from "@/lib/e621";
+
+interface E621State {
+  credentials: E621Credentials | null;
+  loading: boolean;
+  load: () => Promise<void>;
+  save: (values: {
+    username: string;
+    api_key: string;
+    base_url: string;
+  }) => Promise<E621Credentials>;
+  clear: () => void;
+}
+
+export const useE621 = create<E621State>((set) => ({
+  credentials: null,
+  loading: false,
+
+  load: async () => {
+    set({ loading: true });
+    try {
+      const credentials = await api<E621Credentials>("/api/auth/e621/");
+      set({ credentials, loading: false });
+    } catch {
+      set({ credentials: null, loading: false });
+    }
+  },
+
+  save: async (values) => {
+    const credentials = await api<E621Credentials>("/api/auth/e621/", {
+      method: "PUT",
+      json: values,
+    });
+    set({ credentials });
+    return credentials;
+  },
+
+  clear: () => set({ credentials: null }),
+}));