Snip/backend/internal/httpx/frontend/server.go

// Package frontend serves the built SPA. It owns exactly the app routes
// (/, /login, /dashboard) plus static assets; everything else is 404 here and
// handled by the redirect command in production.
package frontend

import (
	"net/http"
	"os"
	"path"
	"path/filepath"
)

type Server struct {
	dist     string
	fs       http.Handler
	spa      map[string]bool
	indexAbs string
}

func New(dist string) *Server {
	return &Server{
		dist:     dist,
		fs:       http.FileServer(http.Dir(dist)),
		spa:      map[string]bool{"/": true, "/login": true, "/dashboard": true},
		indexAbs: filepath.Join(dist, "index.html"),
	}
}

func (s *Server) Handler() http.Handler {
	mux := http.NewServeMux()
	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Content-Type", "application/json")
		_, _ = w.Write([]byte(`{"status":"ok"}`))
	})
	mux.HandleFunc("/", s.serve)
	return mux
}

func (s *Server) serve(w http.ResponseWriter, r *http.Request) {
	clean := path.Clean(r.URL.Path)

	// App routes always render the SPA shell.
	if s.spa[clean] {
		http.ServeFile(w, r, s.indexAbs)
		return
	}

	// Real static file (assets, favicon, …)?
	if clean != "/" {
		full := filepath.Join(s.dist, filepath.FromSlash(clean))
		if st, err := os.Stat(full); err == nil && !st.IsDir() {
			s.fs.ServeHTTP(w, r)
			return
		}
	}

	// Unknown path: not this command's concern.
	http.NotFound(w, r)
}