Add comprehensive group customization features

- Add group overlay color and opacity settings
- Add font customization (body and heading fonts)
- Add group description text color control
- Add option to hide 'Groups' title
- Update frontend DesignSettings interface
- Update CustomizationPanel with new UI controls
- Update Django model with new fields
- Create migration for new customization options
- Update DRF serializer with validation

frontend/linktree-frontend/src/app/(protected)/dashboard/DashboardClient.tsx

@@ -61,6 +61,14 @@ interface DesignSettings {
   cover_overlay_enabled?: boolean
   cover_overlay_color?: string
   cover_overlay_opacity?: number
+  // Новые опции кастомизации
+  group_overlay_enabled?: boolean
+  group_overlay_color?: string
+  group_overlay_opacity?: number
+  show_groups_title?: boolean
+  group_description_text_color?: string
+  body_font_family?: string
+  heading_font_family?: string
 }
 
 export default function DashboardClient() {