Add comprehensive blog demo showcasing advanced content management features

- Implement complete mushroom foraging blog with chanterelles article
- Add rich demonstration of .insertr and .insertr-add functionality
- Include comprehensive documentation for future .insertr-content vision
- Update project styling and configuration to support blog demo
- Enhance engine and API handlers for improved content management

internal/engine/engine.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/insertr/insertr/internal/db"
 	"golang.org/x/net/html"
+	"slices"
 )
 
 // AuthProvider represents authentication provider information
@@ -118,7 +119,7 @@ func (e *ContentEngine) ProcessContent(input ContentInput) (*ContentResult, erro
 		}
 	}
 
-	// 5. Inject content if required by mode
+	// 6. Inject content if required by mode
 	if input.Mode == Enhancement || input.Mode == ContentInjection {
 		err = e.injectContent(processedElements, input.SiteID)
 		if err != nil {
@@ -170,7 +171,8 @@ func (e *ContentEngine) findEditableElements(doc *html.Node) ([]InsertrElement,
 						Node: n,
 					})
 				}
-			} else if e.hasInsertrAddClass(n) {
+			}
+			if e.hasInsertrAddClass(n) {
 				// Collection element - add directly (no container transformation for collections)
 				collectionElements = append(collectionElements, CollectionElement{
 					Node: n,
@@ -197,16 +199,6 @@ func (e *ContentEngine) findEditableElements(doc *html.Node) ([]InsertrElement,
 	return insertrElements, collectionElements
 }
 
-// findInsertrElements finds all elements with class="insertr" and applies container transformation
-// This implements the "syntactic sugar transformation" from CLASSES.md:
-// - Containers with .insertr get their .insertr class removed
-// - Viable children of those containers get .insertr class added
-// - Regular elements with .insertr are kept as-is
-func (e *ContentEngine) findInsertrElements(doc *html.Node) []InsertrElement {
-	insertrElements, _ := e.findEditableElements(doc)
-	return insertrElements
-}
-
 // walkNodes walks through all nodes in the document
 func (e *ContentEngine) walkNodes(n *html.Node, fn func(*html.Node)) {
 	fn(n)
@@ -218,23 +210,13 @@ func (e *ContentEngine) walkNodes(n *html.Node, fn func(*html.Node)) {
 // hasInsertrClass checks if node has class="insertr"
 func (e *ContentEngine) hasInsertrClass(node *html.Node) bool {
 	classes := GetClasses(node)
-	for _, class := range classes {
-		if class == "insertr" {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(classes, "insertr")
 }
 
 // hasInsertrAddClass checks if node has class="insertr-add" (collection)
 func (e *ContentEngine) hasInsertrAddClass(node *html.Node) bool {
 	classes := GetClasses(node)
-	for _, class := range classes {
-		if class == "insertr-add" {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(classes, "insertr-add")
 }
 
 // addContentAttributes adds data-content-id attribute only