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/vite.config.ts

@@ -36,6 +36,8 @@ export default defineConfig({
             '@radix-ui/react-slot',
             '@radix-ui/react-switch',
           ],
+          // Markdown rendering
+          'vendor-markdown': ['react-markdown', 'remark-gfm'],
           // Icons and utilities
           'vendor-utils': [
             'lucide-react',