feat(app): add virtual scrolling for chat messages

Replace v-for message loop with @tanstack/vue-virtual useVirtualizer.
Dynamic row measurement, estimated sizes (60px user / 200px assistant),
overscan 5 items. Scroll-to-bottom via scrollToIndex during streaming.

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

packages/app/package.json

@@ -20,6 +20,7 @@
   },
   "dependencies": {
     "@aiui/core": "workspace:*",
+    "@tanstack/vue-virtual": "^3.13.19",
     "markdown-it": "^14.1.1",
     "pinia": "^3.0.4",
     "plyr": "^3.8.4",