xrdwb17xu: Add the full-screen snap feed with media and quote slides on phones

src/components/layout-context.ts

@@ -0,0 +1,22 @@
+"use client";
+
+import { createContext, useContext } from "react";
+
+/** Widest `.app` width that gets the phone layout (spec "Mobile layout", ADR 0001 section 3). */
+export const COMPACT_MAX_WIDTH = 760;
+
+/** True when `.app` is 760px wide or less. Matches `@container app (max-width: 760px)`. */
+export function isCompactWidth(width: number): boolean {
+  return width > 0 && width <= COMPACT_MAX_WIDTH;
+}
+
+/**
+ * Whether the signed-in shell is in the phone layout. AppShell measures `.app`
+ * and provides it; outside the shell (tests, other pages) it is false, so the
+ * desktop markup renders.
+ */
+export const CompactLayoutContext = createContext(false);
+
+export function useCompactLayout(): boolean {
+  return useContext(CompactLayoutContext);
+}