Migrate to Chi router and add Norwegian Devigo demo

Major improvements:
- Replace Gorilla Mux with Chi v5 router for better performance and cleaner code
- Fix CSS/JS MIME type issues that prevented proper asset loading
- Add built-in CORS middleware replacing manual OPTIONS handlers
- Simplify routing with nested route syntax
- Update URL parameter extraction from mux.Vars to chi.URLParam

New Devigo demo:
- Add production Norwegian sales training website (devigo.no)
- Real-world Hugo-generated content with TailwindCSS
- 39 insertr-enhanced elements for comprehensive CMS testing
- Demonstrates international language support and B2B use cases
- Fixed asset paths for localhost serving compatibility

Technical benefits:
- Automatic MIME type detection for static files
- Reduced code complexity with built-in middleware
- Better performance with lighter dependency stack
- Production-ready CORS handling

internal/api/handlers.go

@@ -13,7 +13,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/gorilla/mux"
+	"github.com/go-chi/chi/v5"
 	"github.com/insertr/insertr/internal/auth"
 	"github.com/insertr/insertr/internal/content"
 	"github.com/insertr/insertr/internal/db"
@@ -97,8 +97,7 @@ func (h *ContentHandler) EnhanceSite(w http.ResponseWriter, r *http.Request) {
 
 // GetContent handles GET /api/content/{id}
 func (h *ContentHandler) GetContent(w http.ResponseWriter, r *http.Request) {
-	vars := mux.Vars(r)
-	contentID := vars["id"]
+	contentID := chi.URLParam(r, "id")
 	siteID := r.URL.Query().Get("site_id")
 
 	if siteID == "" {
@@ -366,8 +365,7 @@ func (h *ContentHandler) CreateContent(w http.ResponseWriter, r *http.Request) {
 
 // DeleteContent handles DELETE /api/content/{id}
 func (h *ContentHandler) DeleteContent(w http.ResponseWriter, r *http.Request) {
-	vars := mux.Vars(r)
-	contentID := vars["id"]
+	contentID := chi.URLParam(r, "id")
 	siteID := r.URL.Query().Get("site_id")
 
 	if siteID == "" {
@@ -403,8 +401,7 @@ func (h *ContentHandler) DeleteContent(w http.ResponseWriter, r *http.Request) {
 
 // GetContentVersions handles GET /api/content/{id}/versions
 func (h *ContentHandler) GetContentVersions(w http.ResponseWriter, r *http.Request) {
-	vars := mux.Vars(r)
-	contentID := vars["id"]
+	contentID := chi.URLParam(r, "id")
 	siteID := r.URL.Query().Get("site_id")
 
 	if siteID == "" {
@@ -456,8 +453,7 @@ func (h *ContentHandler) GetContentVersions(w http.ResponseWriter, r *http.Reque
 
 // RollbackContent handles POST /api/content/{id}/rollback
 func (h *ContentHandler) RollbackContent(w http.ResponseWriter, r *http.Request) {
-	vars := mux.Vars(r)
-	contentID := vars["id"]
+	contentID := chi.URLParam(r, "id")
 	siteID := r.URL.Query().Get("site_id")
 
 	if siteID == "" {