Add a Pools browser backed by e621's /pools endpoint

- /pools: search by name, category/active filters, sort options and
  pagination per the OpenAPI spec, with covers taken from each pool's
  first post in one batched post call; blacklisted covers fall back to a
  placeholder and deleted pools get an archive marker.
- /pools/<id>: DText description, post grid kept in the pool's own order
  with chunked loading, in-library badges, a blacklist reveal toggle and
  a Follow pool button wired into the follows API.
- e621 client gains fetchPools/fetchPool; Pools nav entry added.

frontend/src/App.tsx

@@ -20,6 +20,8 @@ import FollowedPage from "@/features/follows/FollowedPage";
 import LibraryPage from "@/features/library/LibraryPage";
 import Md5Redirect from "@/features/library/Md5Redirect";
 import OnlinePage from "@/features/online/OnlinePage";
+import PoolDetailPage from "@/features/pools/PoolDetailPage";
+import PoolsPage from "@/features/pools/PoolsPage";
 import UploadPage from "@/features/upload/UploadPage";
 import UsersPage from "@/features/users/UsersPage";
 import { isAgeVerified, markAgeVerified } from "@/lib/age";
@@ -71,6 +73,8 @@ export default function App() {
         <Route path="/view/:md5" element={<Md5Redirect />} />
         <Route path="/online" element={<OnlinePage />} />
         <Route path="/online/view/:postId" element={<PostRedirect />} />
+        <Route path="/pools" element={<PoolsPage />} />
+        <Route path="/pools/:poolId" element={<PoolDetailPage />} />
         <Route
           path="/followed"
           element={