Add a viewer-first mobile shell that swaps in under 768px without restyling desktop Bench.

Co-authored-by: Cursor <cursoragent@cursor.com>

composables/useIsMobile.ts

@@ -0,0 +1,29 @@
+/** Portrait shell gate. SSR defaults desktop to avoid hydration flip. */
+export function useIsMobile() {
+  const isMobile = useState('studio2.isMobile', () => false)
+
+  function read() {
+    if (!import.meta.client) return false
+    return window.matchMedia('(max-width: 767px)').matches
+  }
+
+  function sync() {
+    isMobile.value = read()
+  }
+
+  if (import.meta.client) {
+    onMounted(() => {
+      sync()
+      const mq = window.matchMedia('(max-width: 767px)')
+      const onChange = () => { isMobile.value = mq.matches }
+      mq.addEventListener?.('change', onChange)
+      window.addEventListener('resize', sync)
+      onBeforeUnmount(() => {
+        mq.removeEventListener?.('change', onChange)
+        window.removeEventListener('resize', sync)
+      })
+    })
+  }
+
+  return { isMobile }
+}