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/index.ts

@@ -51,3 +51,6 @@ export { PlayIcon, SwitchIcon } from './ActionIcons.js';
 
 // Special icons
 export { ThinkingIcon, TerminalIcon } from './SpecialIcons.js';
+
+// Stop icon
+export { StopIcon } from './StopIcon.js';