feat(managers): table / cards / rows layouts for the channel manager (E4 S3)

A generic ViewSwitcher (reused from the feed) drives a per-account layout
shared by the Subscriptions + Discovery tabs. `table` keeps the DataTable
(numbered pager); `cards` and `rows` render a virtualized grid over the same
client-side list via the new ChannelLayoutGrid, which composes cells straight
from each table's Column<T> render closures (cardPrimary/hideInCard/cardLabel),
so both tabs — with their different columns — get the views for free. Status
chips surface in the fixed band when the table's own controls row is gone.

frontend/src/components/ChannelsProvider.tsx

@@ -8,6 +8,7 @@ import {
   type ReactNode,
 } from "react";
 import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
+import { parseChannelLayout, type ChannelLayout } from "../lib/channelLayout";
 import { useNavigation, useNavigationActions } from "./NavigationProvider";
 import type { ChannelStatusFilter, ChannelsView } from "./Channels";
 
@@ -41,6 +42,8 @@ type ChannelsState = {
   search: string;
   statusFilter: ChannelStatusFilter;
   view: ChannelsView;
+  // The table/cards/rows density (shared by the subscribed + discovery tabs).
+  layout: ChannelLayout;
   // Bumped to tell the manager to drop a stale column filter when an intent sends the user to a
   // specific set (the header's "without full history" link).
   filtersResetToken: number;
@@ -54,6 +57,7 @@ interface ChannelsActions {
   setSearch: (q: string) => void;
   setStatusFilter: (f: ChannelStatusFilter) => void;
   setView: (v: ChannelsView) => void;
+  setLayout: (l: ChannelLayout) => void;
   // "Focus this channel in the manager": jump to Channels (subscriptions tab) and seed its name
   // filter so the channel is isolated. Called from the feed sidebar and the notifications inbox.
   focusChannel: (name: string) => void;
@@ -65,6 +69,7 @@ const StateContext = createContext<ChannelsState>({
   search: "",
   statusFilter: "all",
   view: "subscribed",
+  layout: "table",
   filtersResetToken: 0,
   focusName: null,
   focusToken: 0,
@@ -73,6 +78,7 @@ const ActionsContext = createContext<ChannelsActions>({
   setSearch: () => {},
   setStatusFilter: () => {},
   setView: () => {},
+  setLayout: () => {},
   focusChannel: () => {},
   goToFullHistory: () => {},
 });
@@ -95,6 +101,9 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
   const [view, setViewState] = useState<ChannelsView>(() =>
     clampView(getAccountRaw(LS.channelsView)),
   );
+  const [layout, setLayoutState] = useState<ChannelLayout>(() =>
+    parseChannelLayout(getAccountRaw(LS.channelLayout)),
+  );
   const [filtersResetToken, setFiltersResetToken] = useState(0);
   const [focusName, setFocusName] = useState<string | null>(null);
   const [focusToken, setFocusToken] = useState(0);
@@ -108,6 +117,10 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
     setViewState(v);
     setAccountRaw(LS.channelsView, v);
   }, []);
+  const setLayout = useCallback((l: ChannelLayout) => {
+    setLayoutState(l);
+    setAccountRaw(LS.channelLayout, l);
+  }, []);
 
   const focusChannel = useCallback(
     (name: string) => {
@@ -131,12 +144,12 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
   }, [page]);
 
   const state = useMemo<ChannelsState>(
-    () => ({ search, statusFilter, view, filtersResetToken, focusName, focusToken }),
-    [search, statusFilter, view, filtersResetToken, focusName, focusToken],
+    () => ({ search, statusFilter, view, layout, filtersResetToken, focusName, focusToken }),
+    [search, statusFilter, view, layout, filtersResetToken, focusName, focusToken],
   );
   const actions = useMemo<ChannelsActions>(
-    () => ({ setSearch, setStatusFilter, setView, focusChannel, goToFullHistory }),
-    [setSearch, setStatusFilter, setView, focusChannel, goToFullHistory],
+    () => ({ setSearch, setStatusFilter, setView, setLayout, focusChannel, goToFullHistory }),
+    [setSearch, setStatusFilter, setView, setLayout, focusChannel, goToFullHistory],
   );
 
   return (