feat: decouple hero header from gallery layouts (#158)

- Add separate header_style setting (hero/standard/minimal/none) that can
  be combined with any layout type (grid/masonry/carousel/timeline/mosaic)
- Create HeroHeader and HeroDivider components for reusable hero section
- Add hero_divider_style setting (wave/straight/angle/curve/none)
- Add database migration for header_style and hero_divider_style columns
- Remove deprecated HeroGalleryLayout component
- Fix various TypeScript errors across the codebase:
  - Add missing type properties (css_template_id, updatedAt, justified settings)
  - Fix null handling for event_date and expires_at fields
  - Fix translation function calls and i18n config
  - Remove unused imports and variables

frontend/src/utils/themeMigration.ts

@@ -0,0 +1,61 @@
+import type { ThemeConfig, HeaderStyleType, HeroDividerStyle, GalleryLayoutType } from '../types/theme.types';
+
+/**
+ * Migrates legacy theme configurations that used 'hero' as a galleryLayout
+ * to the new decoupled headerStyle + galleryLayout system.
+ *
+ * This ensures backward compatibility with existing events that have
+ * 'hero' set as their galleryLayout.
+ */
+export function migrateThemeConfig(theme: ThemeConfig): ThemeConfig {
+  if (!theme) return theme;
+
+  // Check if this theme uses the legacy 'hero' layout
+  if ((theme.galleryLayout as string) === 'hero') {
+    return {
+      ...theme,
+      headerStyle: 'hero' as HeaderStyleType,
+      galleryLayout: 'grid' as GalleryLayoutType,
+      heroDividerStyle: (theme.heroDividerStyle || 'wave') as HeroDividerStyle,
+    };
+  }
+
+  // If headerStyle is not set but galleryLayout is valid, default to 'standard'
+  if (!theme.headerStyle && theme.galleryLayout) {
+    return {
+      ...theme,
+      headerStyle: 'standard' as HeaderStyleType,
+    };
+  }
+
+  return theme;
+}
+
+/**
+ * Parses and migrates a color_theme JSON string from the database.
+ * Handles both JSON strings and legacy preset names.
+ */
+export function parseAndMigrateTheme(colorTheme: string | null | undefined): ThemeConfig | null {
+  if (!colorTheme) return null;
+
+  try {
+    // Check if it's a JSON string
+    if (colorTheme.startsWith('{')) {
+      const parsed = JSON.parse(colorTheme);
+      return migrateThemeConfig(parsed);
+    }
+
+    // Legacy preset name - return null to let the caller handle preset lookup
+    return null;
+  } catch {
+    // Invalid JSON
+    return null;
+  }
+}
+
+/**
+ * Checks if a theme configuration needs migration from legacy hero layout.
+ */
+export function needsMigration(theme: ThemeConfig): boolean {
+  return (theme.galleryLayout as string) === 'hero';
+}