fix(vscode-ide-companion): fix send/stop button state timing issue

- Show stop button immediately when request is submitted (isWaitingForResponse=true)
- Prevent confusing state where send button appears disabled instead of stop button
- Improve UX by providing immediate visual feedback on request submission

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/vscode-ide-companion/src/webview/App.tsx

@@ -624,6 +624,7 @@ export const App: React.FC = () => {
         inputText={inputText}
         inputFieldRef={inputFieldRef}
         isStreaming={messageHandling.isStreaming}
+        isWaitingForResponse={messageHandling.isWaitingForResponse}
         isComposing={isComposing}
         editMode={editMode}
         thinkingEnabled={thinkingEnabled}