feat(managers): paginate the card layout + its top scroll-fade

UAT round 3 feedback on the card layout:
- Add a pager + "Cards per page" to the card view (it infinitely-scrolled before).
  Extracted the pager cluster out of DataTable into a reusable Pager component
  (the table now renders it too — no behaviour change); the card view slices the
  sorted rows and drives its own per-account page size (subscribed + discovery).
- Restore the top edge-fade in the card layout. The channels page registers
  fadeTop:false for its sticky-header table; the card layout has no sticky header,
  so App now re-enables the top fade when the channel layout is cards.

frontend/src/App.tsx

@@ -27,6 +27,7 @@ import {
 } from "./lib/storage";
 import { useNavigation, useNavigationActions } from "./components/NavigationProvider";
 import { useFeedFilters, useFeedFiltersActions } from "./components/FeedFiltersProvider";
+import { useChannels } from "./components/ChannelsProvider";
 import { useFeedView, useFeedViewActions } from "./components/FeedViewProvider";
 import { moduleDef } from "./lib/modules";
 import { PAGE_CONTENT, PAGE_RAIL } from "./components/moduleRegistry";
@@ -62,6 +63,9 @@ export default function App() {
   // handlers that mutate the feed view (onShowInFeed, tag-click, the feed scrollKey).
   const filters = useFeedFilters();
   const { setFilters } = useFeedFiltersActions();
+  // The channel manager turns its top fade OFF for the table (its sticky header sits at the scroller
+  // top) but wants it ON for the card layout (no sticky header) — so the fade depends on the layout.
+  const { layout: channelLayout } = useChannels();
   // The feed's view mode (card/row/tile density) lives in FeedViewProvider — server-synced, shared
   // by the feed page and the channel page. App reads it only to hand to <Feed>/<ChannelPage>.
   const view = useFeedView();
@@ -387,7 +391,9 @@ export default function App() {
         ) : (
           <PageShell
             scrollKey={normalScrollKey}
-            fadeTop={fadeTop ?? true}
+            // Channels: the registry disables the top fade for its sticky-header table, but the card
+            // layout has no sticky header, so re-enable it there.
+            fadeTop={activePage === "channels" ? channelLayout === "cards" : fadeTop ?? true}
             header={<Header me={meQuery.data!} onYtSearch={enterYtSearch} />}
             banners={
               <>