Fix Yjs WebSocket in dev mode

- Add Vite plugin implementing y-protocols sync server for dev mode
- Fix useYjs hook: move provider creation into useEffect to survive
  React StrictMode remounts
- Fix WebSocket URL: use /sync as base (y-websocket appends room name)
- Verified in browser: Connected status, Host badge, awareness working

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

apps/planner/vite.config.ts

@@ -2,9 +2,10 @@ import { reactRouter } from "@react-router/dev/vite";
 import tailwindcss from "@tailwindcss/vite";
 import { defineConfig } from "vite";
 import path from "node:path";
+import { yjsDevPlugin } from "./app/lib/vite-yjs-plugin";
 
 export default defineConfig({
-  plugins: [tailwindcss(), reactRouter()],
+  plugins: [tailwindcss(), reactRouter(), yjsDevPlugin()],
   resolve: {
     alias: {
       "~": path.resolve(__dirname, "./app"),