fix: show teacher introductions as standalone clickable bubbles

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

@@ -2221,6 +2221,9 @@ test('teacher introductions collapse after three operation replies and remain av
     await page.evaluate(() => { window.location.hash = '/chat'; });
     const introductions = page.getByTestId('teacher-welcomes');
     await expect(introductions.locator('.teacher-welcome-card')).toHaveCount(2);
+    await expect(introductions).toHaveCSS('background-color', 'rgba(0, 0, 0, 0)');
+    await expect(introductions.getByText('来认识一下')).toHaveCount(0);
+    await expect(introductions.getByRole('button', { name: '先自己试试' })).toHaveCount(0);
     const startedAt = await page.evaluate(() => Date.now());
     for (let round = 1; round <= 3; round++) {
       for (const status of ['running', 'idle'] as const) {