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/injector.go

@@ -9,7 +9,6 @@ import (
 	"github.com/insertr/insertr/internal/config"
 	"github.com/insertr/insertr/internal/db"
 	"golang.org/x/net/html"
-	"slices"
 )
 
 // Injector handles content injection into HTML elements
@@ -168,8 +167,8 @@ func (i *Injector) findElementByTag(node *html.Node, tag string) *html.Node {
 
 // AddContentAttributes adds necessary data attributes and insertr class for editor functionality
 func (i *Injector) AddContentAttributes(node *html.Node, contentID string, contentType string) {
-	i.setAttribute(node, "data-content-id", contentID)
-	i.addClass(node, "insertr")
+	SetAttribute(node, "data-content-id", contentID)
+	AddClass(node, "insertr")
 }
 
 // InjectEditorAssets adds editor JavaScript to HTML document and injects demo gate if needed
@@ -200,63 +199,6 @@ func (i *Injector) findHeadElement(node *html.Node) *html.Node {
 	return nil
 }
 
-// setAttribute safely sets an attribute on an HTML node
-func (i *Injector) setAttribute(node *html.Node, key, value string) {
-	// Remove existing attribute if present
-	for idx, attr := range node.Attr {
-		if attr.Key == key {
-			node.Attr = slices.Delete(node.Attr, idx, idx+1)
-			break
-		}
-	}
-
-	// Add new attribute
-	node.Attr = append(node.Attr, html.Attribute{
-		Key: key,
-		Val: value,
-	})
-}
-
-// addClass safely adds a class to an HTML node
-func (i *Injector) 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 // Class already exists
-	}
-
-	// 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,
-		})
-	}
-}
-
 // Element represents a parsed HTML element with metadata
 type Element struct {
 	Node    *html.Node