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/lib/e621.ts

@@ -316,6 +316,68 @@ export async function fetchPostsByMd5(
   return data.posts;
 }
 
+export interface E621Pool {
+  id: number;
+  name: string;
+  created_at: string;
+  updated_at: string;
+  creator_id: number;
+  creator_name: string;
+  description: string;
+  is_active: boolean;
+  category: "collection" | "series";
+  post_ids: number[];
+  post_count: number;
+}
+
+export type E621PoolOrder =
+  | "id_desc"
+  | "id_asc"
+  | "name"
+  | "created_at"
+  | "post_count";
+
+export async function fetchPools(
+  credentials: E621Credentials,
+  options: {
+    name?: string;
+    category?: string;
+    isActive?: string;
+    order?: E621PoolOrder;
+    page?: number;
+    limit?: number;
+    signal?: AbortSignal;
+  } = {},
+): Promise<E621Pool[]> {
+  const data = await e621Request<unknown>(credentials, "/pools.json", {
+    params: {
+      limit: options.limit ?? 48,
+      page: options.page,
+      "search[order]": options.order ?? "id_desc",
+      "search[name_matches]": options.name || undefined,
+      "search[category]": options.category || undefined,
+      "search[is_active]": options.isActive || undefined,
+    },
+    signal: options.signal,
+  });
+  // The spec says a bare array; tolerate a wrapped payload too.
+  if (Array.isArray(data)) return data as E621Pool[];
+  const wrapped = (data as { pools?: E621Pool[] })?.pools;
+  return Array.isArray(wrapped) ? wrapped : [];
+}
+
+export async function fetchPool(
+  credentials: E621Credentials,
+  poolId: number,
+  signal?: AbortSignal,
+): Promise<E621Pool> {
+  const data = await e621Request<unknown>(credentials, `/pools/${poolId}.json`, {
+    signal,
+  });
+  const wrapped = (data as { pool?: E621Pool })?.pool;
+  return (wrapped ?? (data as E621Pool)) as E621Pool;
+}
+
 export interface E621IqdbResult {
   post_id: number | null;
   score: number | null;