Add canvas sync bridge for rspace.online integration

PostMessage listener in CanvasEmbed receives shape updates from
the embedded rspace canvas iframe. Canvas-sync utility provides
type guards and API helpers for pushing shapes to rspace.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/CanvasEmbed.tsx

@@ -1,14 +1,31 @@
 'use client';
 
+import { useEffect, useCallback } from 'react';
+import { isCanvasMessage, CanvasShapeMessage } from '@/lib/canvas-sync';
+
 interface CanvasEmbedProps {
   canvasSlug: string;
   className?: string;
+  onShapeUpdate?: (message: CanvasShapeMessage) => void;
 }
 
-export function CanvasEmbed({ canvasSlug, className = '' }: CanvasEmbedProps) {
-  const rspaceUrl = process.env.NEXT_PUBLIC_RSPACE_URL || 'https://rspace.online';
+export function CanvasEmbed({ canvasSlug, className = '', onShapeUpdate }: CanvasEmbedProps) {
   const canvasUrl = `https://${canvasSlug}.rspace.online`;
 
+  const handleMessage = useCallback(
+    (event: MessageEvent) => {
+      if (!isCanvasMessage(event)) return;
+      if (event.data.communitySlug !== canvasSlug) return;
+      onShapeUpdate?.(event.data);
+    },
+    [canvasSlug, onShapeUpdate]
+  );
+
+  useEffect(() => {
+    window.addEventListener('message', handleMessage);
+    return () => window.removeEventListener('message', handleMessage);
+  }, [handleMessage]);
+
   return (
     <div className={`relative bg-slate-900 rounded-xl overflow-hidden border border-slate-700/50 ${className}`}>
       {/* Toolbar */}

src/lib/canvas-sync.ts

@@ -0,0 +1,40 @@
+/**
+ * Canvas sync utilities for rtrips.online <-> rspace.online integration
+ *
+ * Listens for postMessage events from the embedded rspace canvas iframe
+ * and provides methods to push structured data to the canvas.
+ */
+
+export interface CanvasShapeMessage {
+  source: 'rspace-canvas';
+  type: 'shape-updated' | 'shape-deleted';
+  communitySlug: string;
+  shapeId: string;
+  data: Record<string, unknown>;
+}
+
+export function isCanvasMessage(event: MessageEvent): event is MessageEvent<CanvasShapeMessage> {
+  return event.data?.source === 'rspace-canvas';
+}
+
+/**
+ * Push shapes to an rspace canvas community via the shapes API
+ */
+export async function pushShapesToCanvas(
+  canvasSlug: string,
+  shapes: Record<string, unknown>[],
+  rspaceUrl?: string
+): Promise<void> {
+  const baseUrl = rspaceUrl || process.env.RSPACE_INTERNAL_URL || 'http://rspace-online:3000';
+
+  const response = await fetch(`${baseUrl}/api/communities/${canvasSlug}/shapes`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ shapes }),
+  });
+
+  if (!response.ok) {
+    const text = await response.text();
+    throw new Error(`Failed to push shapes: ${response.status} ${text}`);
+  }
+}