fix: move middleware to handlers, as it belongs there

handler/middleware/coop.go

@@ -0,0 +1,13 @@
+package middleware
+
+import (
+	"net/http"
+)
+
+func Coop(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Cross-Origin-Opener-Policy", "same-origin")
+
+		next.ServeHTTP(w, r)
+	})
+}