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

internal/api/handlers.go

@@ -731,3 +731,30 @@ func (h *ContentHandler) ServeInsertrJS(w http.ResponseWriter, r *http.Request)
 	// Copy file contents to response
 	io.Copy(w, file)
 }
+
+// ServeInsertrCSS handles GET /insertr.css - serves the insertr CSS stylesheet
+func (h *ContentHandler) ServeInsertrCSS(w http.ResponseWriter, r *http.Request) {
+	// Path to the built insertr.css file
+	cssPath := "lib/dist/insertr.css"
+
+	// Check if file exists
+	if _, err := os.Stat(cssPath); os.IsNotExist(err) {
+		http.Error(w, "insertr.css not found - run 'just build-lib' to build the library", http.StatusNotFound)
+		return
+	}
+
+	// Open and serve the file
+	file, err := os.Open(cssPath)
+	if err != nil {
+		http.Error(w, fmt.Sprintf("Failed to open insertr.css: %v", err), http.StatusInternalServerError)
+		return
+	}
+	defer file.Close()
+
+	// Set appropriate headers
+	w.Header().Set("Content-Type", "text/css")
+	w.Header().Set("Cache-Control", "no-cache") // For development
+
+	// Copy file contents to response
+	io.Copy(w, file)
+}