handlers: a directory under /s/ answers 404, not a listing (closes #286)

http.FileServer answered /s/ and /s/css/ with an HTML listing of the
embedded files, and /s/css with a redirect to that listing. HandleStatic
in internal/handlers now looks the name up in the embedded files first
and answers 404 when it names a directory; files are served as before.
The routes mount it under /s.

Model: opus-5-5

internal/handlers/static.go

@@ -0,0 +1,36 @@
+package handlers
+
+import (
+	"io/fs"
+	"net/http"
+	"strings"
+
+	"sneak.berlin/go/dnswatcher/static"
+)
+
+// HandleStatic returns the handler for the embedded static assets. It
+// is mounted under /s, with that prefix stripped from the path. A
+// request that names a directory answers 404: http.FileServer would
+// answer it with a listing of the directory, or with a redirect to
+// that listing when the trailing slash is missing.
+func (h *Handlers) HandleStatic() http.HandlerFunc {
+	files := http.FileServer(http.FS(static.Static))
+
+	return func(
+		writer http.ResponseWriter,
+		request *http.Request,
+	) {
+		// fs.Stat refuses a name that is empty or ends in a slash,
+		// so /s/ and /s/css/ answer 404 here along with /s/css.
+		name := strings.TrimPrefix(request.URL.Path, "/")
+
+		info, err := fs.Stat(static.Static, name)
+		if err != nil || info.IsDir() {
+			http.NotFound(writer, request)
+
+			return
+		}
+
+		files.ServeHTTP(writer, request)
+	}
+}