Fix following past 48 entries and make the e621 page size configurable

Follow lists were paginated at the API default of 48, but the SPA treats
them as complete sets: the tag/pool toggles read their state from page one
(so the 49th follow looked unfollowed and its spinner waited for a page that
could never contain it) and the Followed page rendered only 48 cards while
showing that as the count. Both follow endpoints are now unpaginated — they
are per-user sets and still restricted to the caller's rows — and the three
consumers take plain arrays.

Post visibility: the old J621-Django online view fetched limit=320 (e621's
maximum) while ours hard-coded 48, and fetchPostsByIds capped id batches at
100. The Online browser now has a 'Posts per page' setting (48/100/200/320)
in its sidebar, mirrored in Account -> Browsing preferences, stored per user
as e621_per_page and also used for pool loading; the id-batch cap is raised
to 320.

Tests: follow list shape/isolation (4) and preference validation/merge (3)
added; the full backend suite is 46 green. Live-checked the array response
shape and the preference bounds (200 accepted, 500 rejected).

frontend/src/store/ui.ts

@@ -16,6 +16,9 @@ interface UiState {
   /** Minimum thumbnail width in px; media grids derive their column count. */
   zoom: number;
   setZoom: (zoom: number) => void;
+  /** Posts per e621 request for the Online browser and pool loading (max 320). */
+  e621PerPage: number;
+  setE621PerPage: (value: number) => void;
 }
 
 export const useUi = create<UiState>()(
@@ -39,6 +42,9 @@ export const useUi = create<UiState>()(
 
       zoom: 190,
       setZoom: (zoom) => set({ zoom }),
+
+      e621PerPage: 48,
+      setE621PerPage: (e621PerPage) => set({ e621PerPage }),
     }),
     { name: "j621.ui" },
   ),