Consolidate DOM manipulation utilities to eliminate code duplication

- Move addClass and setAttribute from ContentEngine/Injector to utils.go
- Remove duplicate hasInsertrClass implementation
- Add RemoveClass and HasClass utilities for completeness
- Eliminates 74+ lines of exact duplication across files

internal/engine/utils.go

@@ -32,6 +32,112 @@ func GetAttribute(node *html.Node, key string) string {
 	return ""
 }
 
+// SetAttribute sets an attribute on an HTML node
+func SetAttribute(node *html.Node, key, value string) {
+	// Check for existing attribute and update in place
+	for i, attr := range node.Attr {
+		if attr.Key == key {
+			node.Attr[i].Val = value
+			return
+		}
+	}
+	// Add new attribute if not found
+	node.Attr = append(node.Attr, html.Attribute{
+		Key: key,
+		Val: value,
+	})
+}
+
+// AddClass safely adds a class to an HTML node
+func AddClass(node *html.Node, className string) {
+	var classAttr *html.Attribute
+	var classIndex int = -1
+
+	// Find existing class attribute
+	for idx, attr := range node.Attr {
+		if attr.Key == "class" {
+			classAttr = &attr
+			classIndex = idx
+			break
+		}
+	}
+
+	var classes []string
+	if classAttr != nil {
+		classes = strings.Fields(classAttr.Val)
+	}
+
+	// Check if class already exists
+	if slices.Contains(classes, className) {
+		return
+	}
+
+	// Add new class
+	classes = append(classes, className)
+	newClassValue := strings.Join(classes, " ")
+
+	if classIndex >= 0 {
+		// Update existing class attribute
+		node.Attr[classIndex].Val = newClassValue
+	} else {
+		// Add new class attribute
+		node.Attr = append(node.Attr, html.Attribute{
+			Key: "class",
+			Val: newClassValue,
+		})
+	}
+}
+
+// RemoveClass safely removes a class from an HTML node
+func RemoveClass(node *html.Node, className string) {
+	var classIndex int = -1
+
+	// Find existing class attribute
+	for idx, attr := range node.Attr {
+		if attr.Key == "class" {
+			classIndex = idx
+			break
+		}
+	}
+
+	if classIndex == -1 {
+		return // No class attribute found
+	}
+
+	// Parse existing classes
+	classes := strings.Fields(node.Attr[classIndex].Val)
+
+	// Filter out the target class
+	var newClasses []string
+	for _, class := range classes {
+		if class != className {
+			newClasses = append(newClasses, class)
+		}
+	}
+
+	// Update or remove class attribute
+	if len(newClasses) == 0 {
+		// Remove class attribute entirely if no classes remain
+		node.Attr = slices.Delete(node.Attr, classIndex, classIndex+1)
+	} else {
+		// Update class attribute with remaining classes
+		node.Attr[classIndex].Val = strings.Join(newClasses, " ")
+	}
+}
+
+// HasClass checks if a node has a specific class
+func HasClass(node *html.Node, className string) bool {
+	for _, attr := range node.Attr {
+		if attr.Key == "class" {
+			classes := strings.Fields(attr.Val)
+			if slices.Contains(classes, className) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 // Inline formatting elements that are safe for editing
 var inlineFormattingTags = map[string]bool{
 	"strong": true,