feat(managers): actions-first discovery table, tabs in the back chain, pager scrolls to top

UAT round 4:
- Discovery table: move Actions to the first column, mirroring the Subscriptions
  table (where it sits right after Prio) so both tabs' actions line up.
- The manager's tabs now ride in history.state (`_chTab`), the same way the channel
  page (`_chan`) and the live search (`_yt`) already do — so Back steps back through
  the tabs you visited before it leaves the module. Arriving from elsewhere stamps
  the entry setPage pushed (one navigation never costs two Back presses).
- Turning a page now scrolls the list back to its top (both the tables and the card
  grid, since they share Pager) — landing mid-list in an unseen page is disorienting.

frontend/src/components/Pager.tsx

@@ -34,9 +34,17 @@ export default function Pager({
   // Local text for the editable "jump to page" box (committed on Enter/blur).
   const [pageInput, setPageInput] = useState(String(safePage + 1));
   useEffect(() => setPageInput(String(safePage + 1)), [safePage]);
+
+  // Turning the page swaps the whole list, so put the reader at its top — landing mid-list in a page
+  // you haven't seen is disorienting. The page scroller is the app's <main> (the same implicit
+  // contract BackToTop and VirtualGrid's getScrollParent rely on).
+  function goToPage(p: number) {
+    onPageChange(p);
+    document.querySelector("main")?.scrollTo({ top: 0 });
+  }
   function commitPageInput() {
     const n = parseInt(pageInput, 10);
-    if (!isNaN(n)) onPageChange(Math.min(totalPages - 1, Math.max(0, n - 1)));
+    if (!isNaN(n)) goToPage(Math.min(totalPages - 1, Math.max(0, n - 1)));
     else setPageInput(String(safePage + 1));
   }
 
@@ -46,7 +54,7 @@ export default function Pager({
       {totalPages > 1 && (
         <div className="flex items-center gap-2">
           <button
-            onClick={() => onPageChange(Math.max(0, safePage - 1))}
+            onClick={() => goToPage(Math.max(0, safePage - 1))}
             disabled={safePage === 0}
             className="glass-card glass-hover flex items-center gap-1 px-3 py-1.5 rounded-lg disabled:opacity-40 transition"
           >
@@ -69,7 +77,7 @@ export default function Pager({
             {t("datatable.pager.ofTotal", { total: totalPages })}
           </span>
           <button
-            onClick={() => onPageChange(Math.min(totalPages - 1, safePage + 1))}
+            onClick={() => goToPage(Math.min(totalPages - 1, safePage + 1))}
             disabled={safePage >= totalPages - 1}
             className="glass-card glass-hover flex items-center gap-1 px-3 py-1.5 rounded-lg disabled:opacity-40 transition"
           >