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/ChannelsProvider.tsx

@@ -120,9 +120,19 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
     setStatusFilterState(f);
     setAccountRaw(LS.channelFilter, f);
   }, []);
+  // The active tab rides in history.state (`_chTab`), the same way the channel page (`_chan`) and the
+  // live YouTube search (`_yt`) do — so Back steps back through the tabs you actually visited instead
+  // of jumping straight out to the previous module.
   const setView = useCallback((v: ChannelsView) => {
     setViewState(v);
     setAccountRaw(LS.channelsView, v);
+    const st = window.history.state || {};
+    // Only own an entry while the manager is already on screen. Arriving from elsewhere
+    // (focusChannel / goToFullHistory) lets setPage push the entry — the stamp effect labels it — so
+    // one navigation never costs two Back presses.
+    if (st.sfPage === "channels") {
+      window.history.pushState({ ...st, sfPage: "channels", _chTab: v }, "");
+    }
   }, []);
   const setLayout = useCallback((l: ChannelLayout) => {
     setLayoutState(l);
@@ -154,6 +164,27 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
     if (page !== "channels") setFocusName(null);
   }, [page]);
 
+  // Label the entry the manager was REACHED on (setPage pushes a bare { sfPage }), so stepping back
+  // onto it later restores the tab it belonged to rather than leaving whatever tab is current.
+  useEffect(() => {
+    if (page !== "channels") return;
+    const st = window.history.state || {};
+    if (st._chTab === undefined) window.history.replaceState({ ...st, _chTab: view }, "");
+  }, [page, view]);
+
+  // Back/Forward across tab entries: restore the tab the entry carries.
+  useEffect(() => {
+    function onPop(e: PopStateEvent) {
+      const tab = e.state?._chTab;
+      if (!tab) return; // an entry outside the manager — leave the tab alone
+      const next = clampView(tab as string);
+      setViewState(next);
+      setAccountRaw(LS.channelsView, next);
+    }
+    window.addEventListener("popstate", onPop);
+    return () => window.removeEventListener("popstate", onPop);
+  }, []);
+
   const state = useMemo<ChannelsState>(
     () => ({ search, statusFilter, view, layout, sort, filtersResetToken, focusName, focusToken }),
     [search, statusFilter, view, layout, sort, filtersResetToken, focusName, focusToken],