chore(slideshow): renumber migrations to 138/139 (after whatsapp #649's 137)

PR #649 takes migration 137 (add_whatsapp_template_language). Renumber the
slideshow migrations to slot in after it:
- 137_add_slideshow_share.js  -> 138_add_slideshow_share.js
- 138_add_slideshow_styling.js -> 139_add_slideshow_styling.js
and update the slideshow migration-number references in comments/types. No
content change — both are additive + addColumnIfNotExists-guarded, so re-running
under the new filename on an already-migrated DB is a safe no-op.

frontend/src/components/admin/SlideshowSettingsCard.tsx

@@ -1,7 +1,7 @@
 /**
  * <SlideshowSettingsCard>
  *
- * Per-event "Live Slideshow" ("Diashow") admin surface (migrations 137/138).
+ * Per-event "Live Slideshow" ("Diashow") admin surface (migrations 138/139).
  * A token-only fullscreen kiosk link for live events that auto-picks-up new
  * uploads while it runs. Mounted once on the EventDetailsPage; admin can:
  *   - Generate the slideshow link on demand (mints show_share_token)

frontend/src/pages/admin/EventDetailsPage.tsx

@@ -2159,7 +2159,7 @@ export const EventDetailsPage: React.FC = () => {
             </div>
           </Card>
 
-          {/* Live Slideshow ("Diashow") link + live display settings (migrations 137/138).
+          {/* Live Slideshow ("Diashow") link + live display settings (migrations 138/139).
               Gated behind the `slideshow` feature flag. */}
           {flags.slideshow && (
           <SlideshowSettingsCard

frontend/src/pages/admin/__tests__/brandingCustomCss.test.tsx

@@ -0,0 +1,90 @@
+import { render, screen, waitFor } from '@testing-library/react';
+import { vi } from 'vitest';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+
+import { GALLERY_THEME_PRESETS } from '../../../types/theme.types';
+
+const CSS = '.gallery-premium-footer p:first-child { display: none; }';
+const savedTheme = { ...GALLERY_THEME_PRESETS.galleryPremium.config, customCss: CSS };
+
+// --- mocks -----------------------------------------------------------------
+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('react-toastify', () => ({ toast: { success: vi.fn(), error: vi.fn() } }));
+
+vi.mock('../../../services/settings.service', async () => {
+  const actual = await vi.importActual<any>('../../../services/settings.service');
+  return {
+    ...actual,
+    settingsService: {
+      ...actual.settingsService,
+      getSettingsByType: vi.fn(async (type: string) =>
+        type === 'theme' ? { theme_config: savedTheme } : {}
+      ),
+      getAllSettings: vi.fn(async () => ({ thumbnail_width: '800', thumbnail_height: '800' })),
+      updateTheme: vi.fn(async () => undefined),
+      updateBranding: vi.fn(async () => undefined),
+    },
+  };
+});
+
+vi.mock('../../../services/fonts.service', () => ({
+  fontsService: { list: vi.fn(async () => []) },
+  extractFamilyName: (s: string) => s,
+}));
+
+vi.mock('../../../services/businessProfile.service', () => ({
+  businessProfileService: { get: vi.fn(async () => ({ profile: {} })), update: vi.fn() },
+}));
+
+vi.mock('../../../hooks/usePublicSettings', () => ({
+  PUBLIC_SETTINGS_QUERY_KEY: ['public-settings'],
+  usePublicSettings: () => ({ data: { branding_force_color_mode: null } }),
+}));
+
+vi.mock('../../../contexts/FeatureFlagsContext', () => ({
+  useFeatureFlags: () => ({ flags: {} }),
+  useFeatureEnabled: () => false,
+}));
+
+// Heavy/unrelated children — stub to keep the test focused on the CSS textarea.
+vi.mock('../../../components/admin/CustomerDashboardBrandingCard', () => ({
+  CustomerDashboardBrandingCard: () => null,
+}));
+vi.mock('../../../components/admin/PdfTypographyCard', () => ({
+  PdfTypographyCard: () => null,
+}));
+vi.mock('../../../components/admin', async () => {
+  const actual = await vi.importActual<any>('../../../components/admin');
+  return { ...actual, GalleryPreview: () => null };
+});
+
+import { BrandingPage } from '../BrandingPage';
+import { ThemeProvider } from '../../../contexts/ThemeContext';
+
+describe('BrandingPage custom CSS persistence (#645)', () => {
+  beforeEach(() => localStorage.clear());
+
+  it('shows the saved custom CSS in the textarea on load for a Gallery Premium theme', async () => {
+    const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+    render(
+      <QueryClientProvider client={qc}>
+        <ThemeProvider>
+          <BrandingPage />
+        </ThemeProvider>
+      </QueryClientProvider>
+    );
+
+    const textarea = (await screen.findByPlaceholderText(
+      '/* Add custom CSS here */'
+    )) as HTMLTextAreaElement;
+
+    await waitFor(() => expect(textarea.value).toBe(CSS));
+  });
+});

frontend/src/types/index.ts

@@ -67,7 +67,7 @@ export interface Event {
   // Client access (#172)
   client_access_enabled?: boolean;
   client_share_token?: string;
-  // Live Slideshow / "Diashow" (migration 137). Token-only fullscreen kiosk
+  // Live Slideshow / "Diashow" (migration 138). Token-only fullscreen kiosk
   // link minted on demand; null token = disabled. Settings drive the running
   // projector and can be changed live.
   show_share_token?: string | null;