feat: Day 14-17 - Frontend Chat Interface completed

Frontend:
- Create MessageList component with streaming animation
- Create MessageInput component with @knowledge base support
- Create ModelSelector component (DeepSeek/Qwen/Gemini)
- Implement conversationApi with SSE streaming
- Update AgentChatPage integrate all components
- Add Markdown rendering (react-markdown + remark-gfm)
- Add code highlighting (react-syntax-highlighter)
- Add vite-env.d.ts for environment variables

Features:
- Real-time streaming output with cursor animation
- Markdown and code block rendering
- Model switching (DeepSeek-V3, Qwen3-72B, Gemini Pro)
- @Knowledge base selector (UI ready)
- Auto-scroll to bottom
- Shift+Enter for new line, Enter to send
- Beautiful message bubble design

Build: Frontend build successfully (7.94s, 1.9MB)

New Files:
- components/chat/MessageList.tsx (170 lines)
- components/chat/MessageList.css (150 lines)
- components/chat/MessageInput.tsx (145 lines)
- components/chat/MessageInput.css (60 lines)
- components/chat/ModelSelector.tsx (110 lines)
- components/chat/ModelSelector.css (35 lines)
- api/conversationApi.ts (170 lines)
- src/vite-env.d.ts (9 lines)

Total: ~850 lines of new code

frontend/package.json

@@ -12,12 +12,16 @@
   "dependencies": {
     "@ant-design/icons": "^5.5.2",
     "@types/js-yaml": "^4.0.9",
+    "@types/react-syntax-highlighter": "^15.5.13",
     "antd": "^5.22.5",
     "axios": "^1.12.2",
     "js-yaml": "^4.1.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
-    "react-router-dom": "^6.28.0"
+    "react-markdown": "^10.1.0",
+    "react-router-dom": "^6.28.0",
+    "react-syntax-highlighter": "^15.6.6",
+    "remark-gfm": "^4.0.1"
   },
   "devDependencies": {
     "@types/react": "^18.3.18",