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/api/handlers.go

@@ -421,11 +421,16 @@ func (h *ContentHandler) CreateCollectionItem(w http.ResponseWriter, r *http.Req
 		return
 	}
 
-	if req.SiteID == "" || req.CollectionID == "" {
-		req.SiteID = r.URL.Query().Get("site_id")
+	// Set collection ID from URL param if not provided in body
+	if req.CollectionID == "" {
 		req.CollectionID = collectionID
 	}
 
+	// Set site ID from query param if not provided in body
+	if req.SiteID == "" {
+		req.SiteID = r.URL.Query().Get("site_id")
+	}
+
 	if req.SiteID == "" {
 		http.Error(w, "site_id is required", http.StatusBadRequest)
 		return