feat(app): base-aware routing, embedded flag, panel slide animation

- Use import.meta.env.BASE_URL for router history (Archy /aiui/ deployments)
- Capture embedded flag from URL params before router init
- Add panelSlideIn animation with prefers-reduced-motion support

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

packages/app/src/main.ts

@@ -5,8 +5,13 @@ import App from './App.vue'
 import './styles/main.css'
 import { initializePlugins } from './plugins'
 
+// Capture embedded flag before router init (survives SPA navigation)
+// Only embedded when explicitly requested via ?embedded param
+const _embeddedFlag = new URLSearchParams(window.location.search).has('embedded')
+;(window as unknown as Record<string, unknown>).__AIUI_EMBEDDED__ = _embeddedFlag
+
 const router = createRouter({
-  history: createWebHistory(),
+  history: createWebHistory(import.meta.env.BASE_URL),
   routes: [
     {
       path: '/',