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

README.md

@@ -910,6 +910,7 @@ internal/
     templates/dashboard.html   Dashboard template (embedded)
     status.go                  /api/v1/status
     healthcheck.go             Health check handler
+    static.go                  Static assets under /s/
   server/
     server.go                  HTTP server lifecycle
     routes.go                  Route definitions