fix: correct dependencies and add Dockerfile

- Mark direct dependencies properly in go.mod (fsnotify, fpdf, goldmark)
- Remove duplicate Theme assignment in type_webserver.go
- Add Dockerfile for multi-stage build (was missing from repo)
- Update PLAN.md with completed tasks

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

type_webserver.go

@@ -29,7 +29,6 @@ func (s *WebServer) Initialize() {
 	}
 	s.AppName = "Go Template Container Web Server"
 	s.Theme = "default"
-	s.Theme = "default"
 
 	// Attempt to read the config file (try both config.yml and config.yaml)
 	var configFile []byte