fix: keep the welcome composer at the bottom

tests/e2e/pi-coding-first-chat.spec.ts

@@ -1110,6 +1110,9 @@ test('first PI Conversation is editable under 500 ms and submits before runtime
       await appendFile(performanceFragment, `${JSON.stringify({ editableMs })}\n`);
     }
 
+    await expect(page.getByTestId('coding-message-composer')).toBeInViewport({ ratio: 1 });
+    expect(await page.getByTestId('coding-message-composer').evaluate(el => innerHeight - el.getBoundingClientRect().bottom)).toBeLessThan(40);
+
     const pixelPng = Buffer.from(
       'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII=',
       'base64',