topbar: surface publish warnings instead of discarding them

handlePublish's JSON response has always included a `warnings` array
(e.g. the contact-form relay's "submissions will not be delivered"
notice), but nothing in the editor ever read it. Adds a PublishWarnings
banner, held in its own state independent of the 3s publishStatus
flash so the customer has time to read it, rendered in both the
desktop and mobile TopBar branches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/panels/topbar/PublishWarnings.test.tsx

@@ -0,0 +1,57 @@
+import { describe, test, expect, vi, afterEach } from 'vitest';
+import React from 'react';
+import { createRoot, Root } from 'react-dom/client';
+import { act } from 'react-dom/test-utils';
+import { PublishWarnings } from './PublishWarnings';
+
+/* ---------- DOM test harness (no @testing-library/react in this repo, see
+   src/ui/AssetPicker.test.tsx for the same pattern: react-dom/client +
+   react-dom/test-utils `act`, both transitive deps of react-dom already). ---------- */
+let container: HTMLDivElement;
+let root: Root;
+
+function render(ui: React.ReactElement) {
+  container = document.createElement('div');
+  document.body.appendChild(container);
+  act(() => {
+    root = createRoot(container);
+    root.render(ui);
+  });
+}
+
+function click(el: Element | null) {
+  if (!el) throw new Error('element not found');
+  act(() => { (el as HTMLElement).dispatchEvent(new MouseEvent('click', { bubbles: true })); });
+}
+
+afterEach(() => {
+  if (container) {
+    act(() => { root.unmount(); });
+    container.remove();
+  }
+});
+
+describe('PublishWarnings', () => {
+  test('renders nothing when there are no warnings', () => {
+    render(<PublishWarnings warnings={[]} onDismiss={() => {}} />);
+    expect(container.innerHTML).toBe('');
+  });
+
+  test('renders each warning', () => {
+    render(
+      <PublishWarnings
+        warnings={['First problem.', 'Second problem.']}
+        onDismiss={() => {}}
+      />,
+    );
+    expect(container.textContent).toContain('First problem.');
+    expect(container.textContent).toContain('Second problem.');
+  });
+
+  test('dismiss fires the callback', () => {
+    const onDismiss = vi.fn();
+    render(<PublishWarnings warnings={['A problem.']} onDismiss={onDismiss} />);
+    click(container.querySelector('[data-testid="publish-warnings-dismiss"]'));
+    expect(onDismiss).toHaveBeenCalledTimes(1);
+  });
+});