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/App.tsx

@@ -3,6 +3,7 @@ import { Navigate, Route, Routes, useLocation } from "react-router-dom";
 
 import { AppShell } from "@/components/AppShell";
 import { FullPageSpinner } from "@/components/ui";
+import AccountPage from "@/features/account/AccountPage";
 import LoginPage from "@/features/auth/LoginPage";
 import RegisterPage from "@/features/auth/RegisterPage";
 import FileDetailPage from "@/features/library/FileDetailPage";
@@ -44,6 +45,14 @@ export default function App() {
             </RequireAuth>
           }
         />
+        <Route
+          path="/account"
+          element={
+            <RequireAuth>
+              <AccountPage />
+            </RequireAuth>
+          }
+        />
         <Route path="*" element={<Navigate to="/" replace />} />
       </Route>
       <Route path="/login" element={<LoginPage />} />