fix(feed): give the view switcher the keyboard model its menu role promises

Arrows/Home/End move a roving focus and Escape hands focus back to the
trigger — the menu unmounts under the focused item, which stranded focus
on <body>. Outside-click deliberately does not refocus: that would steal
focus from whatever was just clicked.

frontend/src/lib/feedView.ts

@@ -4,9 +4,10 @@
 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 {
+const FEED_VIEW_DEFAULT: FeedView = "cards";
+
+function isFeedView(v: unknown): v is FeedView {
   return typeof v === "string" && (FEED_VIEWS as readonly string[]).includes(v);
 }