fix(dashboard): grant read access to storageclasses for authenticated users

The dashboard UI fetches StorageClasses via the Kubernetes API to populate
dropdowns (e.g. in the Postgres form), but the cozystack-dashboard-readonly
ClusterRole did not include storage.k8s.io/storageclasses. This caused
authenticated users to see "Error" instead of the StorageClass name.

Add get/list/watch permissions for storageclasses to the dashboard readonly
role, consistent with the existing backupclasses entry.

Assisted-By: Claude AI
Signed-off-by: Kirill Ilin <stitch14@yandex.ru>

packages/system/dashboard/templates/rbac.yaml

@@ -20,6 +20,14 @@ rules:
   - get
   - list
   - watch
+- apiGroups:
+  - storage.k8s.io
+  resources:
+  - storageclasses
+  verbs:
+  - get
+  - list
+  - watch
 ---
 apiVersion: rbac.authorization.k8s.io/v1
 kind: ClusterRoleBinding