Add color overlay settings for link buttons

- Added link_overlay_enabled, link_overlay_color, link_overlay_opacity to DesignSettings model
- Created migration 0008 for new fields
- Updated CustomizationPanel with link overlay controls section
- Added validation for new overlay settings in serializer
- Updated PublicDesignSettingsSerializer to include overlay settings
- Applied link overlay to all ExpandableGroup components in public page
- Added preview in customization panel for link overlay effect

backend/customization/migrations/0009_merge_20251109_0356.py

@@ -0,0 +1,14 @@
+# Generated by Django 5.2.7 on 2025-11-09 03:56
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('customization', '0008_add_template_id_and_test_list_layout'),
+        ('customization', '0008_designsettings_link_overlay_color_and_more'),
+    ]
+
+    operations = [
+    ]