feat(vscode-ide-companion): refactor message components with modular architecture

Refactor UI message rendering by extracting message types into dedicated components. Add ChatHeader component for better session management interface.

- Extract message components: UserMessage, AssistantMessage, ThinkingMessage, StreamingMessage, WaitingMessage
- Add ChatHeader component with session selector and action buttons
- Delete MessageContent.css and consolidate styles into App.scss
- Update Tailwind config for component styling
- Improve message rendering with proper TypeScript typing

packages/vscode-ide-companion/src/webview/components/messages/index.tsx

@@ -0,0 +1,11 @@
+/**
+ * @license
+ * Copyright 2025 Qwen Team
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+export { UserMessage } from './UserMessage.js';
+export { AssistantMessage } from './AssistantMessage.js';
+export { ThinkingMessage } from './ThinkingMessage.js';
+export { StreamingMessage } from './StreamingMessage.js';
+export { WaitingMessage } from './WaitingMessage.js';