feat(app): add markdown rendering in chat messages

Install markdown-it and render assistant messages through it with safe
defaults (html: false, linkify, breaks). Add CSS for code blocks,
lists, links, blockquotes, headings. Links open in new tab. User
messages remain plain text.

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

packages/app/package.json

@@ -20,6 +20,7 @@
   },
   "dependencies": {
     "@aiui/core": "workspace:*",
+    "markdown-it": "^14.1.1",
     "pinia": "^3.0.4",
     "plyr": "^3.8.4",
     "vue": "^3.5.29",
@@ -29,6 +30,7 @@
     "@eslint/js": "^10.0.1",
     "@playwright/test": "^1.49.0",
     "@tailwindcss/vite": "^4.2.1",
+    "@types/markdown-it": "^14.1.2",
     "@vitejs/plugin-vue": "^6.0.4",
     "duck-duck-scrape": "^2.2.7",
     "eslint": "^10.0.2",