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")
 }

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{

internal/db/repository.go

@@ -18,6 +18,7 @@ type ContentRepository interface {
 	CreateCollection(ctx context.Context, siteID, collectionID, containerHTML, lastEditedBy string) (*CollectionItem, error)
 	GetCollectionItems(ctx context.Context, siteID, collectionID string) ([]CollectionItemWithTemplate, error)
 	GetCollectionTemplates(ctx context.Context, siteID, collectionID string) ([]CollectionTemplateItem, error)
+	GetCollectionTemplate(ctx context.Context, templateID int) (*CollectionTemplateItem, error)
 	CreateCollectionTemplate(ctx context.Context, siteID, collectionID, name, htmlTemplate string, isDefault bool) (*CollectionTemplateItem, error)
 	CreateCollectionItem(ctx context.Context, siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*CollectionItemWithTemplate, error)
 	CreateCollectionItemAtomic(ctx context.Context, siteID, collectionID string, templateID int, lastEditedBy string) (*CollectionItemWithTemplate, error)

internal/db/sqlite_repository.go

@@ -219,6 +219,24 @@ func (r *SQLiteRepository) GetCollectionTemplates(ctx context.Context, siteID, c
 	return result, nil
 }
 
+// GetCollectionTemplate retrieves a single template by ID
+func (r *SQLiteRepository) GetCollectionTemplate(ctx context.Context, templateID int) (*CollectionTemplateItem, error) {
+	template, err := r.queries.GetCollectionTemplate(ctx, int64(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 != 0, // SQLite uses INTEGER for boolean
+	}
+	return result, nil
+}
+
 // CreateCollectionItem creates a new collection item
 func (r *SQLiteRepository) CreateCollectionItem(ctx context.Context, siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*CollectionItemWithTemplate, error) {
 	item, err := r.queries.CreateCollectionItem(ctx, sqlite.CreateCollectionItemParams{