Refactor: Immutable item enrichment in mobile-menu and nav-menu

Replace mutate-in-map with spread-in-map so enrichment helpers don't
mutate the caller's input. Two callsites surfaced under freeze-on-set:

- mobile-menu addNavIcons: was mutating item.navIcon on items from
  settings.menu, silently modifying the caller's menu tree
- nav-menu addSelectedIndex: was mutating item.selectedIndex on items
  from settings.menu, same issue; inner .map passes now use the
  returned value instead of relying on side effects

Author: jlukic

src/components/mobile-menu/mobile-menu.js

@@ -100,16 +100,13 @@ const createComponent = ({ tpl, settings, $, state, flush, afterFlush, dispatchE
 
   // add -> icon if the menu has a sub menu
   addNavIcons(menu) {
-    if (!settings.navIcon) {
+    if (!settings.navIcon || !menu?.menu) {
       return menu;
     }
-    (menu?.menu || []).map(item => {
-      if (item.menu) {
-        item.navIcon = settings.navIcon;
-      }
-      return item;
-    });
-    return menu;
+    return {
+      ...menu,
+      menu: menu.menu.map(item => item.menu ? { ...item, navIcon: settings.navIcon } : item),
+    };
   },
   addTrailingSlash(url) {
     return (url.substr(-1) === '/')

src/components/nav-menu/nav-menu.js

@@ -147,25 +147,30 @@ const createComponent = ({ $, el, self, settings, state, reaction, isRendered })
     let firstMatch = false;
     const searchTermChanged = self.lastSearchTerm !== searchTerm;
     const addSelectedIndex = (item) => {
-      if (item?.url) {
-        selectedIndex++;
-        item.selectedIndex = selectedIndex;
+      if (!item?.url) {
+        return item;
       }
+      selectedIndex++;
       // reset selected index only when search term changes
-      if (searchTermChanged && !firstMatch && item.highlight && item?.url) {
+      if (searchTermChanged && !firstMatch && item.highlight) {
         state.selectedIndex.set(selectedIndex);
         firstMatch = true;
       }
-      return item;
+      return { ...item, selectedIndex };
     };
     // add selected index for each menu, pages and subpages (3 deep max)
     menu = menu.map(currentMenu => {
-      currentMenu = addSelectedIndex(currentMenu);
-      (currentMenu?.pages || []).map(page => {
-        page = addSelectedIndex(page);
-        (page?.pages || []).map(subPage => addSelectedIndex(subPage));
+      const withIndex = addSelectedIndex(currentMenu);
+      if (!withIndex?.pages) { return withIndex; }
+      const pages = withIndex.pages.map(page => {
+        const pageWithIndex = addSelectedIndex(page);
+        if (!pageWithIndex?.pages) { return pageWithIndex; }
+        return {
+          ...pageWithIndex,
+          pages: pageWithIndex.pages.map(subPage => addSelectedIndex(subPage)),
+        };
       });
-      return currentMenu;
+      return { ...withIndex, pages };
     });
     self.lastSearchTerm = searchTerm;
     state.maxIndex.set(selectedIndex);