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/lib/e621.ts

@@ -255,7 +255,7 @@ export async function fetchPostsByIds(
     credentials,
     "/posts.json",
     {
-      params: { tags: `id:${ids.join(",")}`, limit: Math.min(ids.length, 100) },
+      params: { tags: `id:${ids.join(",")}`, limit: Math.min(ids.length, 320) },
       signal,
     },
   );

frontend/src/lib/preferences.ts

@@ -12,6 +12,9 @@ export function applyPreferences(
   if (typeof preferences.ordering === "string") ui.setOrdering(preferences.ordering);
   if (typeof preferences.per_page === "number") ui.setPerPage(preferences.per_page);
   if (typeof preferences.zoom === "number") ui.setZoom(preferences.zoom);
+  if (typeof preferences.e621_per_page === "number") {
+    ui.setE621PerPage(preferences.e621_per_page);
+  }
 }
 
 /** The browse settings currently in effect, as a preferences payload. */
@@ -22,6 +25,7 @@ export function preferencesPayload(extra?: UserPreferences): UserPreferences {
     ordering: ui.ordering,
     per_page: ui.perPage,
     zoom: ui.zoom,
+    e621_per_page: ui.e621PerPage,
     ...extra,
   };
 }

frontend/src/lib/types.ts

@@ -33,6 +33,8 @@ export interface UserPreferences {
   per_page?: number;
   zoom?: number;
   online_hot_default?: boolean;
+  /** Posts per e621 request for the Online browser and pool loading (max 320). */
+  e621_per_page?: number;
 }
 
 export interface User {