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/server/routes.go

@@ -8,8 +8,6 @@ import (
 	"github.com/go-chi/chi/v5"
 	chimw "github.com/go-chi/chi/v5/middleware"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
-
-	"sneak.berlin/go/dnswatcher/static"
 )
 
 // requestTimeout is the maximum duration for handling a request.
@@ -47,13 +45,7 @@ func (s *Server) SetupRoutes() {
 	public.Get("/", s.handlers.HandleDashboard())
 
 	// Static assets (embedded CSS/JS)
-	public.Mount(
-		"/s",
-		http.StripPrefix(
-			"/s",
-			http.FileServer(http.FS(static.Static)),
-		),
-	)
+	public.Mount("/s", http.StripPrefix("/s", s.handlers.HandleStatic()))
 
 	// Health check (standard well-known path)
 	public.Get(

internal/server/routes_test.go

@@ -1,6 +1,7 @@
 package server_test
 
 import (
+	"mime"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -288,3 +289,42 @@ func TestMetricsRateLimitComesBeforeAuth(t *testing.T) {
 		t.Errorf("another address: status = %d, want 200", rec.Code)
 	}
 }
+
+// TestStaticDirectoriesAnswerNotFound checks that a request naming a
+// directory of the static assets, with or without the trailing slash,
+// answers 404 rather than a listing of it or a redirect to one, and
+// that the stylesheet is still served as CSS.
+func TestStaticDirectoriesAnswerNotFound(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+
+	srv := routedServer(t)
+
+	for _, path := range []string{"/s", "/s/", "/s/css", "/s/css/"} {
+		rec := serve(srv, httptest.NewRequestWithContext(
+			t.Context(), http.MethodGet, path, nil,
+		))
+
+		if rec.Code != http.StatusNotFound {
+			t.Errorf("GET %s: status = %d, want 404", path, rec.Code)
+		}
+	}
+
+	rec := serve(srv, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/s/css/tailwind.min.css", nil,
+	))
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("GET stylesheet: status = %d, want 200", rec.Code)
+	}
+
+	contentType := rec.Header().Get("Content-Type")
+
+	mediaType, _, err := mime.ParseMediaType(contentType)
+	if err != nil || mediaType != "text/css" {
+		t.Errorf(
+			"GET stylesheet: Content-Type = %q, want text/css",
+			contentType,
+		)
+	}
+}