feat: implement serve mode with file watching

- Add fsnotify for file watching in serve mode
- Create watcher.go for content directory monitoring
- Update serve mode to generate initial output and watch for changes
- Add route to serve generated HTML files from output directory
- File changes trigger automatic regeneration

Co-Authored-By: Claude (glm-5) <noreply@anthropic.com>

routes.go

@@ -1,7 +1,14 @@
 package main
 
-import "github.com/gorilla/mux"
+import (
+	"net/http"
+
+	"github.com/gorilla/mux"
+)
 
 func (s *WebServer) Routes(r *mux.Router) {
 	r.HandleFunc("/version", handleVersion).Methods("GET")
+
+	// Serve generated HTML files from output directory
+	r.PathPrefix("/").Handler(http.StripPrefix("/", http.FileServer(http.Dir(outputPath))))
 }