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/channelLayout.ts

@@ -0,0 +1,33 @@
+// The channel manager's layout vocabulary (E4 S3). Named `ChannelLayout` — NOT `ChannelView` — on
+// purpose: the manager already has a `ChannelsView` (the subscribed/discovery/blocked TAB), and a
+// one-letter-apart type would be a footgun. This is the table-vs-card-vs-row density switch, the
+// sibling of the feed's `FeedView`; it rides the same generic `ViewSwitcher`.
+export type ChannelLayout = "table" | "cards" | "rows";
+
+export const CHANNEL_LAYOUTS: readonly ChannelLayout[] = ["table", "cards", "rows"] as const;
+
+const CHANNEL_LAYOUT_DEFAULT: ChannelLayout = "table";
+
+/** The card/row layouts' grid spec, consumed by VirtualGrid (S3.3). `table` has no entry — it keeps
+ * the numbered-pager DataTable, not the virtualized grid. Tuned by measuring the real layouts. */
+export interface ChannelLayoutSpec {
+  /** Narrowest grid column in px, or null for ONE full-width column (the row list). */
+  minCol: number | null;
+  /** Single-column reading-width cap in px (null = fill). */
+  maxCol: number | null;
+  /** First-paint row-height estimate; real heights are measured per-row after. */
+  rowEst: number;
+}
+
+export const CHANNEL_LAYOUT_SPEC: Record<Exclude<ChannelLayout, "table">, ChannelLayoutSpec> = {
+  cards: { minCol: 300, maxCol: null, rowEst: 210 },
+  rows: { minCol: null, maxCol: 1100, rowEst: 88 },
+};
+
+/** Coerce a stored value to a layout; anything unknown (corrupt, a rolled-back future layout) falls
+ * back to the default rather than rendering nothing. */
+export function parseChannelLayout(raw: unknown): ChannelLayout {
+  return (CHANNEL_LAYOUTS as readonly string[]).includes(raw as string)
+    ? (raw as ChannelLayout)
+    : CHANNEL_LAYOUT_DEFAULT;
+}

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",