feat(app): activate plugin registry with Claude provider adapter

Create plugins/index.ts bootstrap and claude-provider.ts implementing
the AIProviderAdapter interface from @aiui/core. The Claude provider
wraps the existing proxy streaming logic as an async generator. Plugin
initialization is called in main.ts before app mount.

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

packages/app/src/main.ts

@@ -3,6 +3,7 @@ import { createPinia } from 'pinia'
 import { createRouter, createWebHistory } from 'vue-router'
 import App from './App.vue'
 import './styles/main.css'
+import { initializePlugins } from './plugins'
 
 const router = createRouter({
   history: createWebHistory(),
@@ -23,4 +24,9 @@ const router = createRouter({
 const app = createApp(App)
 app.use(createPinia())
 app.use(router)
+
+initializePlugins().catch((err) => {
+  console.error('[AIUI] Failed to initialize plugins:', err)
+})
+
 app.mount('#app')