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(