feat(feed): pick the feed's view from its own toolbar

The view mode moves out of Settings into a switcher next to sort, on the
feed and channel pages alike. It leaves the Settings prefs draft for its
own eager save: a toolbar click must not mark that page dirty and trip
its leave-page guard.

lib/feedView.ts owns the vocabulary and maps the old grid/list prefs
forward on read, so nobody's saved choice is lost.

frontend/src/lib/feedView.ts

@@ -0,0 +1,21 @@
+// The feed's view vocabulary. Lifted out of Feed (like feedSort.ts) because it has three
+// consumers that can't share module-private state: App owns the pref, the toolbar's
+// ViewSwitcher offers it, and VirtualFeed/VideoCard render it.
+export type FeedView = "cards" | "rows";
+
+export const FEED_VIEWS: readonly FeedView[] = ["cards", "rows"] as const;
+export const FEED_VIEW_DEFAULT: FeedView = "cards";
+
+export function isFeedView(v: unknown): v is FeedView {
+  return typeof v === "string" && (FEED_VIEWS as readonly string[]).includes(v);
+}
+
+/** Coerce a stored pref to a view. Up to 0.44 it was "grid" | "list"; those map forward here so
+ * nobody's saved choice is lost. Anything else (a corrupt value, a future view rolled back) falls
+ * back to the default rather than rendering nothing. */
+export function parseFeedView(raw: unknown): FeedView {
+  if (isFeedView(raw)) return raw;
+  if (raw === "grid") return "cards";
+  if (raw === "list") return "rows";
+  return FEED_VIEW_DEFAULT;
+}