fix: float teacher introductions above a centered welcome

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

@@ -1113,6 +1113,11 @@ test('first PI Conversation is editable under 500 ms and submits before runtime
     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);
     await expect(page.getByTestId('community-invitation')).toBeVisible();
+    expect(await page.getByTestId('coding-message-composer').evaluate(el => {
+      const body = document.querySelector('.coding-chat-body')!.getBoundingClientRect();
+      const input = el.getBoundingClientRect();
+      return Math.abs(input.left + input.width / 2 - body.left - body.width / 2);
+    })).toBeLessThan(2);
     expect(await page.getByTestId('community-invitation').evaluate(el => {
       const composer = document.querySelector('[data-testid="coding-message-composer"]')!;
       return composer.getBoundingClientRect().top - el.getBoundingClientRect().bottom;