fix: center bottom composer between sidebar and teachers

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

@@ -1112,6 +1112,11 @@ 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('community-invitation').evaluate(el => {
+      const composer = document.querySelector('[data-testid="coding-message-composer"]')!;
+      return composer.getBoundingClientRect().top - el.getBoundingClientRect().bottom;
+    })).toBeCloseTo(12, 0);
 
     const pixelPng = Buffer.from(
       'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII=',