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/api/handlers.go

@@ -405,6 +405,30 @@ func (h *ContentHandler) GetCollectionItems(w http.ResponseWriter, r *http.Reque
 	json.NewEncoder(w).Encode(items)
 }
 
+// GetCollectionTemplates handles GET /api/collections/{id}/templates
+func (h *ContentHandler) GetCollectionTemplates(w http.ResponseWriter, r *http.Request) {
+	collectionID := chi.URLParam(r, "id")
+	siteID := r.URL.Query().Get("site_id")
+
+	if siteID == "" {
+		http.Error(w, "site_id parameter is required", http.StatusBadRequest)
+		return
+	}
+
+	templates, err := h.repository.GetCollectionTemplates(context.Background(), siteID, collectionID)
+	if err != nil {
+		http.Error(w, fmt.Sprintf("Database error: %v", err), http.StatusInternalServerError)
+		return
+	}
+
+	response := map[string]interface{}{
+		"templates": templates,
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(response)
+}
+
 // CreateCollectionItem handles POST /api/collections/{id}/items
 func (h *ContentHandler) CreateCollectionItem(w http.ResponseWriter, r *http.Request) {
 	userInfo, authErr := h.authService.ExtractUserFromRequest(r)
@@ -437,15 +461,29 @@ func (h *ContentHandler) CreateCollectionItem(w http.ResponseWriter, r *http.Req
 	}
 
 	if req.TemplateID == 0 {
-		req.TemplateID = 1 // Default to first template
+		http.Error(w, "template_id is required", http.StatusBadRequest)
+		return
 	}
 
-	// Use atomic collection item creation from repository
-	createdItem, err := h.repository.CreateCollectionItemAtomic(
-		context.Background(),
+	// Get the specific template by ID
+	selectedTemplate, err := h.repository.GetCollectionTemplate(context.Background(), req.TemplateID)
+	if err != nil {
+		http.Error(w, fmt.Sprintf("Template %d not found: %v", req.TemplateID, err), http.StatusBadRequest)
+		return
+	}
+
+	// Verify template belongs to the requested collection and site
+	if selectedTemplate.CollectionID != req.CollectionID || selectedTemplate.SiteID != req.SiteID {
+		http.Error(w, fmt.Sprintf("Template %d not found in collection %s", req.TemplateID, req.CollectionID), http.StatusBadRequest)
+		return
+	}
+
+	// Use engine's unified collection item creation
+	createdItem, err := h.engine.CreateCollectionItemFromTemplate(
 		req.SiteID,
 		req.CollectionID,
 		req.TemplateID,
+		selectedTemplate.HTMLTemplate,
 		userInfo.ID,
 	)
 	if err != nil {
@@ -487,10 +525,10 @@ func (h *ContentHandler) RegisterRoutes(r chi.Router) {
 		// COLLECTION MANAGEMENT - Groups of related content
 		// =============================================================================
 		r.Route("/collections", func(r chi.Router) {
-			// Public routes
-			r.Get("/", h.GetAllCollections)            // GET /api/collections?site_id=X
-			r.Get("/{id}", h.GetCollection)            // GET /api/collections/{id}?site_id=X
-			r.Get("/{id}/items", h.GetCollectionItems) // GET /api/collections/{id}/items?site_id=X
+			r.Get("/", h.GetAllCollections)                    // GET /api/collections?site_id=X
+			r.Get("/{id}", h.GetCollection)                    // GET /api/collections/{id}?site_id=X
+			r.Get("/{id}/items", h.GetCollectionItems)         // GET /api/collections/{id}/items?site_id=X
+			r.Get("/{id}/templates", h.GetCollectionTemplates) // GET /api/collections/{id}/templates?site_id=X
 
 			// Protected routes
 			r.Group(func(r chi.Router) {