feat(vscode-ide-companion): add cancel streaming functionality

- Add handleCancel callback to App component
- Implement cancelStreaming message posting to VS Code
- Add onCancel prop to InputForm component
- Replace send button with stop button during streaming

packages/vscode-ide-companion/src/webview/components/icons/StopIcon.tsx

@@ -0,0 +1,33 @@
+/**
+ * @license
+ * Copyright 2025 Qwen Team
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Stop icon for canceling operations
+ */
+
+import type React from 'react';
+import type { IconProps } from './types.js';
+
+/**
+ * Stop/square icon (16x16)
+ * Used for stop/cancel operations
+ */
+export const StopIcon: React.FC<IconProps> = ({
+  size = 16,
+  className,
+  ...props
+}) => (
+  <svg
+    xmlns="http://www.w3.org/2000/svg"
+    viewBox="0 0 16 16"
+    fill="currentColor"
+    width={size}
+    height={size}
+    className={className}
+    aria-hidden="true"
+    {...props}
+  >
+    <rect x="4" y="4" width="8" height="8" rx="1" />
+  </svg>
+);