feat: add full markdown rendering to chat messages

Replace the custom BOLD_REGEX parser in ChatMessage.tsx with
react-markdown + remark-gfm for proper rendering of headers, tables,
lists, code blocks, blockquotes, links, and horizontal rules in all
chat UIs (AssistantChat, SpecCreationChat, ExpandProjectChat).

Changes:
- Add react-markdown and remark-gfm dependencies
- Add vendor-markdown chunk to Vite manual chunks for code splitting
- Add .chat-prose CSS class with styles for all markdown elements
- Add .chat-prose-user modifier for contrast on primary-colored bubbles
- Replace line-splitting + regex logic with ReactMarkdown component
- Links open in new tabs via custom component override
- System messages remain plain text (unchanged)

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

ui/package.json

@@ -32,6 +32,8 @@
     "lucide-react": "^0.475.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
+    "react-markdown": "^10.1.0",
+    "remark-gfm": "^4.0.1",
     "tailwind-merge": "^3.4.0"
   },
   "devDependencies": {