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.

ROADMAP.md

@@ -55,6 +55,13 @@ they land.
   - [x] "Followed" nav entry
   - [x] Periodic sync commands: `sync_followed_tags` and
         `sync_followed_pools` (one e621 search per unique tag/pool)
+- [x] **Pools browser** (`/pools` + `/pools/<id>`)
+  - [x] Index search by name; category, active/deleted and sort filters;
+        pagination according to the e621 OpenAPI spec
+  - [x] Cover thumbnails from each pool's first post (one batched post call,
+        blacklist-aware) and deleted-pool markers
+  - [x] Pool detail: DText description, posts kept in pool order with chunked
+        loading and in-library badges, blacklist reveal toggle, Follow button
 - [ ] `F` shortcut to favorite a post, tag finder in the Ctrl+K palette
 
 ## 3. Uploads & ingestion pipeline