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/lib/storage.ts

@@ -29,6 +29,9 @@ export const LS = {
   // The manager's sort, lifted out of the table so the card layout shares it (per-account, per-tab).
   channelSort: "siftlode.channelSort",
   channelDiscoverySort: "siftlode.channelDiscoverySort",
+  // Cards-per-page for the card layout (the table keeps its own rows-per-page in its persist blob).
+  channelCardSize: "siftlode.channelCardSize",
+  channelDiscoveryCardSize: "siftlode.channelDiscoveryCardSize",
   channelsTable: "siftlode.channelsTable",
   channelDiscoveryTable: "siftlode.channelDiscoveryTable",
   settingsTab: "siftlode.settingsTab",