Implement class-based template differentiation and fix collection item creation

- Add class-based template comparison to differentiate styling variants
- Implement template deduplication based on structure + class signatures
- Add GetCollectionTemplate method to repository interface and implementations
- Fix collection item creation by replacing unimplemented CreateCollectionItemAtomic
- Add template selection modal with auto-default selection in frontend
- Generate meaningful template names from distinctive CSS classes
- Fix unique constraint violations with timestamp-based collection item IDs
- Add collection templates API endpoint for frontend template fetching
- Update simple demo with featured/compact/dark testimonial variants for testing

internal/db/postgresql_repository.go

@@ -214,6 +214,24 @@ func (r *PostgreSQLRepository) GetCollectionTemplates(ctx context.Context, siteI
 	return result, nil
 }
 
+// GetCollectionTemplate retrieves a single template by ID
+func (r *PostgreSQLRepository) GetCollectionTemplate(ctx context.Context, templateID int) (*CollectionTemplateItem, error) {
+	template, err := r.queries.GetCollectionTemplate(ctx, int32(templateID))
+	if err != nil {
+		return nil, err
+	}
+
+	result := &CollectionTemplateItem{
+		TemplateID:   int(template.TemplateID),
+		CollectionID: template.CollectionID,
+		SiteID:       template.SiteID,
+		Name:         template.Name,
+		HTMLTemplate: template.HtmlTemplate,
+		IsDefault:    template.IsDefault, // PostgreSQL uses BOOLEAN
+	}
+	return result, nil
+}
+
 // CreateCollectionItem creates a new collection item
 func (r *PostgreSQLRepository) CreateCollectionItem(ctx context.Context, siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*CollectionItemWithTemplate, error) {
 	item, err := r.queries.CreateCollectionItem(ctx, postgresql.CreateCollectionItemParams{