feat(mesh-ui): receive share-to-mesh postMessage + pending attachment

App.vue listens for postMessage({type:'share-to-mesh',cid,...}) from
marketplace app iframes, stashes the payload in sessionStorage, and
routes to /mesh. Mesh.vue reads the stash on mount (and on a synthetic
'archipelago:share-to-mesh' event when already on the view), showing a
pending-attachment banner in the compose area. Send becomes Share and
flushes the CID via mesh.send-content with the input text as caption.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

neode-ui/src/App.vue

@@ -212,6 +212,7 @@ onMounted(async () => {
   window.addEventListener('mousedown', onUserActivity)
   window.addEventListener('keydown', onUserActivity)
   window.addEventListener('touchstart', onUserActivity)
+  window.addEventListener('message', onShareToMeshMessage)
   const seenIntro = localStorage.getItem('neode_intro_seen') === '1'
   const isDirectRoute = route.path !== '/'
   const fromBoot = sessionStorage.getItem('archipelago_from_boot') === '1'
@@ -241,8 +242,31 @@ onBeforeUnmount(() => {
   window.removeEventListener('mousedown', onUserActivity)
   window.removeEventListener('keydown', onUserActivity)
   window.removeEventListener('touchstart', onUserActivity)
+  window.removeEventListener('message', onShareToMeshMessage)
 })
 
+/**
+ * Phase 3c: marketplace app iframes share files into mesh chats by POSTing
+ * to /api/share-to-mesh then postMessaging the CID back to this parent
+ * window. We stash it in sessionStorage + route to /mesh; Mesh.vue reads the
+ * stash on mount and stages it as a pending attachment.
+ */
+function onShareToMeshMessage(ev: MessageEvent) {
+  const data = ev.data as { type?: string; cid?: string } | null
+  if (!data || data.type !== 'share-to-mesh' || !data.cid) return
+  try {
+    sessionStorage.setItem('archipelago_share_to_mesh', JSON.stringify(data))
+  } catch {
+    /* quota — fall through */
+  }
+  if (route.path !== '/mesh') {
+    router.push('/mesh')
+  } else {
+    // Already on /mesh — dispatch a synthetic event so the view picks it up.
+    window.dispatchEvent(new CustomEvent('archipelago:share-to-mesh'))
+  }
+}
+
 /**
  * Handle splash screen completion
  * Routes user directly to appropriate screen based on onboarding status (from backend)