feat: add multilingual chat support with markdown rendering

- Fix Socket.IO proxy configuration in nginx for chat connectivity
- Add Socket.IO path routing (/socket.io/) with WebSocket upgrade support
- Fix frontend healthcheck to use curl instead of wget
- Add react-markdown and remark-gfm for proper markdown rendering
- Implement language selector in chat interface (8 languages supported)
- Add language parameter to chat agent and LLM prompts
- Support English, Italian, Spanish, French, German, Portuguese, Chinese, Japanese

This resolves the chat connection issues and enables users to receive
AI responses in their preferred language with properly formatted markdown.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/package.json

@@ -12,7 +12,8 @@
     "@emotion/react": "^11.11.0",
     "@emotion/styled": "^11.11.0",
     "socket.io-client": "^4.6.0",
-    "markdown-it": "^14.0.0",
+    "react-markdown": "^9.0.1",
+    "remark-gfm": "^4.0.0",
     "date-fns": "^3.0.0"
   },
   "scripts": {