feat(ui): add sessionStorage persistence for TreeView expansion state (R11)

src/ScadaLink.CentralUI/wwwroot/js/treeview-storage.js

@@ -0,0 +1,8 @@
+window.treeviewStorage = {
+    save: function (storageKey, keysJson) {
+        sessionStorage.setItem("treeview:" + storageKey, keysJson);
+    },
+    load: function (storageKey) {
+        return sessionStorage.getItem("treeview:" + storageKey);
+    }
+};