Implement hybrid CSS architecture to fix white-on-white modal issue on sites with CSS resets

- Migrate from inline CSS to external insertr.css with cascade layer architecture
- Add CSS CDN serving capability (ServeInsertrCSS handler and /insertr.css route)
- Implement hybrid approach: @layer insertr for modern browsers + html body selectors for legacy browsers
- Remove scattered inline CSS from JavaScript modules for better maintainability
- Solve form element spacing conflicts with aggressive site CSS resets like '* {margin:0; padding:0}'
- Enable proper CSS caching and separation of concerns

cmd/serve.go

@@ -6,6 +6,7 @@ import (
 	"net/http"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 
 	"github.com/go-chi/chi/v5"
@@ -143,6 +144,7 @@ func runServe(cmd *cobra.Command, args []string) {
 
 	// Static library serving (for demo sites)
 	router.Get("/insertr.js", contentHandler.ServeInsertrJS)
+	router.Get("/insertr.css", contentHandler.ServeInsertrCSS)
 
 	// API routes
 	router.Route("/api", func(apiRouter chi.Router) {
@@ -163,11 +165,21 @@ func runServe(cmd *cobra.Command, args []string) {
 	})
 
 	// Static site serving - serve registered sites at /sites/{site_id}
-	// This fixes the MIME type issues with Chi's FileServer
+	// Custom file server that fixes CSS MIME types
 	for siteID, siteConfig := range siteManager.GetAllSites() {
 		log.Printf("📁 Serving site %s from %s at /sites/%s/", siteID, siteConfig.Path, siteID)
+
+		// Create custom file server with MIME type fixing
 		fileServer := http.FileServer(http.Dir(siteConfig.Path))
-		router.Handle("/sites/"+siteID+"/*", http.StripPrefix("/sites/"+siteID+"/", fileServer))
+		customHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			// Fix MIME type for CSS files (including extensionless ones in css/ directory)
+			if strings.Contains(r.URL.Path, "/css/") {
+				w.Header().Set("Content-Type", "text/css; charset=utf-8")
+			}
+			fileServer.ServeHTTP(w, r)
+		})
+
+		router.Handle("/sites/"+siteID+"/*", http.StripPrefix("/sites/"+siteID+"/", customHandler))
 	}
 
 	// Start server