fix(managers): address the E4 S3 code-review findings

- DataTable: keep `pagerNode` NULLABLE. Extracting Pager made it an always-truthy
  element, so the `controls` / `bottomControls` guards always fired and a table with
  nothing to page rendered empty `my-3` bands (plus an empty in-band PageToolbar).
- ChannelsProvider: `setView` bails when the tab is unchanged (re-selecting the
  active tab, or goToFullHistory forcing "subscribed", stacked dead Back entries),
  and pushes a FRESH state like setPage does instead of spreading `_chan`/`_yt`
  markers into the tab entry.
- Lift the layout into its own narrow context (`useChannelLayout`): App read the
  whole ChannelsState for the fade, so every keystroke in the manager's search box
  re-rendered the app root.
- Reset the card page when the row set or sort changes (both tabs) — the clamp only
  hid a stale page, which came back when the filter was cleared.
- Sort + slice the card rows only in cards mode; table mode was copy-sorting ~300
  rows per render for a result nothing read.
- Validate persisted `cardSize` / `sort` (`parseCardSize`, `parseSortState`), matching
  the clamp-on-read convention the module's other persisted values already follow;
  a corrupt size made the page count NaN and emptied the grid.
- Pager: page-size changes scroll to the top too (they re-slice from a new first item).
- ColumnSortControl: a stored sort naming a column this table no longer offers is
  treated as no sort, instead of showing a direction arrow for a sort that isn't applied.
- Drop the stale "rows" layout comments (that variant was retired this sprint).

frontend/src/App.tsx

@@ -27,7 +27,7 @@ import {
 } from "./lib/storage";
 import { useNavigation, useNavigationActions } from "./components/NavigationProvider";
 import { useFeedFilters, useFeedFiltersActions } from "./components/FeedFiltersProvider";
-import { useChannels } from "./components/ChannelsProvider";
+import { useChannelLayout } from "./components/ChannelsProvider";
 import { useFeedView, useFeedViewActions } from "./components/FeedViewProvider";
 import { moduleDef } from "./lib/modules";
 import { PAGE_CONTENT, PAGE_RAIL } from "./components/moduleRegistry";
@@ -65,7 +65,9 @@ export default function App() {
   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();
+  // Deliberately the NARROW layout context: subscribing to the manager's whole state here would
+  // re-render the app root on every keystroke in its search box.
+  const channelLayout = useChannelLayout();
   // 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();