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/WaitingMessage.tsx

@@ -0,0 +1,31 @@
+/**
+ * @license
+ * Copyright 2025 Qwen Team
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type React from 'react';
+
+interface WaitingMessageProps {
+  loadingMessage: string;
+}
+
+export const WaitingMessage: React.FC<WaitingMessageProps> = ({
+  loadingMessage,
+}) => (
+  <div className="flex gap-0 items-start text-left py-2 flex-col relative opacity-85 animate-[fadeIn_0.2s_ease-in]">
+    <div className="bg-transparent border-0 py-2 flex items-center gap-2">
+      <span className="inline-flex items-center gap-1 mr-0">
+        <span className="inline-block w-1.5 h-1.5 bg-[var(--app-secondary-foreground)] rounded-full mr-0 opacity-60 animate-[typingPulse_1.4s_infinite_ease-in-out] [animation-delay:0s]"></span>
+        <span className="inline-block w-1.5 h-1.5 bg-[var(--app-secondary-foreground)] rounded-full mr-0 opacity-60 animate-[typingPulse_1.4s_infinite_ease-in-out] [animation-delay:0.2s]"></span>
+        <span className="inline-block w-1.5 h-1.5 bg-[var(--app-secondary-foreground)] rounded-full mr-0 opacity-60 animate-[typingPulse_1.4s_infinite_ease-in-out] [animation-delay:0.4s]"></span>
+      </span>
+      <span
+        className="opacity-70 italic"
+        style={{ color: 'var(--app-secondary-foreground)' }}
+      >
+        {loadingMessage}
+      </span>
+    </div>
+  </div>
+);