feat: add Static, StaticFS and SPA file serving

- Add Static(prefix, root) to serve files from disk on Kite and Group
- Add StaticFS(prefix, fs) to serve from any http.FileSystem on Kite and Group
- Add SPA(root) and SPAFS(fs) for Single Page Application serving — falls back to index.html for unknown paths so client-side routing works correctly

group.go

@@ -8,6 +8,21 @@ type Group struct {
 	mws    []Middleware
 }
 
+func (g *Group) Static(prefix, root string) {
+	g.StaticFS(prefix, http.Dir(root))
+}
+
+func (g *Group) StaticFS(prefix string, fs http.FileSystem) {
+	if prefix == "" || prefix[len(prefix)-1] != '/' {
+		prefix += "/"
+	}
+	fullPrefix := g.prefix + prefix
+
+	handler := http.StripPrefix(fullPrefix, http.FileServer(fs))
+	g.k.r.Handler(http.MethodGet, fullPrefix+"*filepath", handler)
+	g.k.r.Handler(http.MethodHead, fullPrefix+"*filepath", handler)
+}
+
 func (g *Group) Group(prefix string, mws ...Middleware) *Group {
 	return &Group{
 		k:      g.k,

kite.go

@@ -130,6 +130,40 @@ func (k *Kite) StartTLS(listenAddr, certFile, keyFile string) error {
 	return srv.Shutdown(ctx)
 }
 
+func (k *Kite) SPA(root string) {
+	k.SPAFS(http.Dir(root))
+}
+
+func (k *Kite) SPAFS(fs http.FileSystem) {
+	fileServer := http.FileServer(fs)
+
+	k.r.NotFound = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		f, err := fs.Open(r.URL.Path)
+		if err != nil {
+			r.URL.Path = "/"
+			fileServer.ServeHTTP(w, r)
+			return
+		}
+		f.Close()
+
+		fileServer.ServeHTTP(w, r)
+	})
+}
+
+func (k *Kite) Static(prefix, root string) {
+	k.StaticFS(prefix, http.Dir(root))
+}
+
+func (k *Kite) StaticFS(prefix string, fs http.FileSystem) {
+	if prefix == "" || prefix[len(prefix)-1] != '/' {
+		prefix += "/"
+	}
+
+	handler := http.StripPrefix(prefix, http.FileServer(fs))
+	k.r.Handler(http.MethodGet, prefix+"*filepath", handler)
+	k.r.Handler(http.MethodHead, prefix+"*filepath", handler)
+}
+
 func (k *Kite) Group(prefix string, mws ...Middleware) *Group {
 	return &Group{
 		k:      k,