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/components/DataTable.tsx

@@ -283,20 +283,24 @@ export default function DataTable<T>({
   // The pager + rows-per-page cluster on its own, so it can be rendered a second time at the bottom
   // (controlsPosition "both") WITHOUT repeating the leading controls (e.g. the status chips) — those
   // are header controls and belong only at the top.
-  const pagerNode = (
-    <Pager
-      page={safePage}
-      size={size}
-      total={sorted.length}
-      pageSizeOptions={pageSizeOptions}
-      onPageChange={setPage}
-      onSizeChange={(s) => {
-        setSize(s);
-        setPage(0);
-      }}
-      sizeLabel={t("datatable.rowsPerPage")}
-    />
-  );
+  // Stays NULLABLE: the wrappers below decide whether to render a controls row at all, and an always
+  // -truthy element would leave an empty `my-3` band (and an empty in-band PageToolbar) on a table
+  // with nothing to page.
+  const pagerNode =
+    totalPages > 1 || sorted.length > 0 ? (
+      <Pager
+        page={safePage}
+        size={size}
+        total={sorted.length}
+        pageSizeOptions={pageSizeOptions}
+        onPageChange={setPage}
+        onSizeChange={(s) => {
+          setSize(s);
+          setPage(0);
+        }}
+        sizeLabel={t("datatable.rowsPerPage")}
+      />
+    ) : null;
 
   const controls =
     controlsLeading != null || pagerNode ? (