fix(admin): portal the update-available modal to document.body

AdminSidebar's root div carries a Tailwind `transform` utility for the mobile
slide-in, and per the CSS spec a transformed ancestor becomes the containing
block for position:fixed descendants. The modal renders inline inside
VersionInfo/AdminSidebar, so its `fixed inset-0` backdrop was trapped in the
256px sidebar column (measured 256 vs window 1440) -- copy buttons overlapping
text, content truncating.

Reuse the codebase's one existing portal convention, from
gallery/FeedbackLimitReachedModal: assign the JSX to a const and return
createPortal(node, document.body).

Checked for other modals with the same trap; there are none.
UpdateInstructionsDialog is also fixed inset-0 but is mounted from
AdminDashboard inside <main>, and CustomerLayout has an identical transformed
aside with no modal inside it.

Refs testplan REPORT.md #13 (Part 3, B.07).

frontend/src/components/admin/UpdateAvailableModal.tsx

@@ -1,4 +1,5 @@
 import React, { useState } from 'react';
+import { createPortal } from 'react-dom';
 import { useQuery } from '@tanstack/react-query';
 import { useTranslation } from 'react-i18next';
 import { X, ExternalLink, Copy, CheckCircle, ChevronDown, ChevronRight, ArrowUpCircle } from 'lucide-react';
@@ -123,7 +124,7 @@ export const UpdateAvailableModal: React.FC<UpdateAvailableModalProps> = ({
     }
   };
 
-  return (
+  const node = (
     <div
       className="fixed inset-0 bg-black/50 flex items-center justify-center z-50 p-4"
       onClick={onClose}
@@ -309,4 +310,11 @@ export const UpdateAvailableModal: React.FC<UpdateAvailableModalProps> = ({
       </Card>
     </div>
   );
+
+  // Portal to body: the modal is rendered from VersionInfo inside the
+  // AdminSidebar, whose root carries a `transform` (mobile slide-in). A
+  // transformed ancestor becomes the containing block for `position: fixed`
+  // descendants, so without this the backdrop sized itself to the 256px
+  // sidebar column instead of the viewport (QA B.07).
+  return createPortal(node, document.body);
 };

frontend/src/components/admin/__tests__/updateAvailableModal.portal.test.tsx

@@ -0,0 +1,62 @@
+/**
+ * UpdateAvailableModal is opened from VersionInfo, which lives inside
+ * AdminSidebar. The sidebar's root carries Tailwind's `transform` utility for
+ * its mobile slide-in, and a transformed ancestor becomes the containing block
+ * for `position: fixed` descendants — so the modal's `fixed inset-0` backdrop
+ * used to size itself to the 256px sidebar column instead of the viewport
+ * (QA B.07). The modal must portal to document.body to escape that.
+ */
+import React from 'react';
+import { describe, it, expect, vi } from 'vitest';
+import { render } from '@testing-library/react';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+
+vi.mock('react-i18next', async () => {
+  const actual = await vi.importActual<typeof import('react-i18next')>('react-i18next');
+  return {
+    ...actual,
+    useTranslation: () => ({ t: (k: string, fb?: unknown) => (typeof fb === 'string' ? fb : k) }),
+  };
+});
+
+vi.mock('../../../config/api', () => ({
+  api: { get: vi.fn().mockResolvedValue({ data: {} }) },
+}));
+
+import { UpdateAvailableModal } from '../UpdateAvailableModal';
+
+const renderModal = () => {
+  const client = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return render(
+    // Stand-in for AdminSidebar's transformed root.
+    <div className="transform" style={{ transform: 'translateX(0)' }}>
+      <QueryClientProvider client={client}>
+        <UpdateAvailableModal
+          currentVersion="1.0.0"
+          latestVersion="1.1.0"
+          onClose={() => {}}
+          onDismiss={() => {}}
+        />
+      </QueryClientProvider>
+    </div>
+  );
+};
+
+describe('UpdateAvailableModal portal (QA B.07)', () => {
+  it('renders the backdrop as a direct child of document.body', () => {
+    renderModal();
+
+    const backdrop = document.querySelector('.fixed.inset-0');
+    expect(backdrop).not.toBeNull();
+    expect(backdrop!.parentElement).toBe(document.body);
+  });
+
+  it('does not render the backdrop inside the transformed sidebar subtree', () => {
+    const { container } = renderModal();
+
+    const transformed = container.querySelector('.transform');
+    expect(transformed).not.toBeNull();
+    expect(transformed!.querySelector('.fixed.inset-0')).toBeNull();
+    expect(container.querySelector('.fixed.inset-0')).toBeNull();
+  });
+});