feat(dc): Complete Tool C Day 5 - AI Chat + Ant Design X Integration

Summary:
- Upgrade to Ant Design 6.0.1 + install Ant Design X (2.1.0) + X SDK (2.1.0)
- Develop frontend common capability layer: Chat component library (~968 lines)
  * ChatContainer.tsx - Core container component
  * MessageRenderer.tsx - Message renderer
  * CodeBlockRenderer.tsx - Code block renderer with syntax highlighting
  * Complete TypeScript types and documentation
- Integrate ChatContainer into Tool C
- Fix 7 critical UI issues:
  * AG Grid module registration error
  * UI refinement (borders, shadows, gradients)
  * Add AI welcome message
  * Auto-clear input field after sending
  * Remove page scrollbars
  * Manual code execution (not auto-run)
  * Support simple Q&A (new /ai/chat API)
- Complete end-to-end testing
- Update all documentation (4 status docs + 6 dev logs)

Technical Stack:
- Frontend: React 19 + Ant Design 6.0 + Ant Design X 2.1
- Components: Bubble, Sender from @ant-design/x
- Total code: ~5418 lines

Status: Tool C MVP completed, production ready

frontend-v2/src/shared/components/index.ts

@@ -0,0 +1,12 @@
+/**
+ * Shared Components - 通用组件统一导出
+ * 
+ * 前端通用能力层组件
+ */
+
+// Chat 组件库
+export * from './Chat';
+
+// 其他通用组件
+export { default as Placeholder } from './Placeholder';
+