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/http_client.go

@@ -192,6 +192,10 @@ func (c *HTTPClient) GetCollectionTemplates(ctx context.Context, siteID, collect
 	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
 }
 
+func (c *HTTPClient) GetCollectionTemplate(ctx context.Context, templateID int) (*CollectionTemplateItem, error) {
+	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
+}
+
 func (c *HTTPClient) CreateCollectionItem(ctx context.Context, siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*CollectionItemWithTemplate, error) {
 	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
 }