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>

go.mod

@@ -7,4 +7,8 @@ require (
 	gopkg.in/yaml.v2 v2.4.0
 )
 
-require github.com/yuin/goldmark v1.7.16 // indirect
+require (
+	github.com/fsnotify/fsnotify v1.9.0 // indirect
+	github.com/yuin/goldmark v1.7.16 // indirect
+	golang.org/x/sys v0.13.0 // indirect
+)