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/lib/storage.ts

@@ -24,6 +24,8 @@ export const LS = {
   feedView: "siftlode.feedView",
   channelFilter: "siftlode.channelFilter",
   channelsView: "siftlode.channelsView",
+  // The channel manager's layout (table / cards / rows — see lib/channelLayout.ts), per-account.
+  channelLayout: "siftlode.channelLayout",
   channelsTable: "siftlode.channelsTable",
   channelDiscoveryTable: "siftlode.channelDiscoveryTable",
   settingsTab: "siftlode.settingsTab",