fix(project-editor): preserve multilingual nav labels on edit

updateLabel() previously overwrote NavigationItemConfig.label with a bare
string via updateNavLink, destroying every other locale's translation
whenever a localized label object was edited. Add a facade method
updateNavLinkLabel() that inspects the existing label shape: plain
strings are replaced as before, but localized objects only have the
current default locale's key overwritten, leaving other locales intact.

src/app/features/project-editor/facade/project-editor.facade.ts

@@ -179,6 +179,38 @@ export class ProjectEditorFacade {
     });
   }
 
+  /**
+   * Updates a nav item's label without destroying other locales' translations.
+   * If the item's current label is a plain string (or unset), it's replaced outright.
+   * If it's already a NavigationLocalizedText object, only the current default
+   * locale's key is overwritten; every other locale key is preserved untouched.
+   */
+  updateNavLinkLabel(target: 'header' | 'footer', id: string, value: string): void {
+    this.updateBootstrap(current => {
+      const list = current.navigation[target];
+      if (!this.isFlatNavList(list)) {
+        return current;
+      }
+      const defaultLocale = current.localization.defaultLocale ?? 'en';
+      return {
+        ...current,
+        navigation: {
+          ...current.navigation,
+          [target]: (list as NavigationItemConfig[]).map(item => {
+            if (item.id !== id) {
+              return item;
+            }
+            const currentLabel = item.label;
+            const nextLabel = currentLabel && typeof currentLabel === 'object'
+              ? { ...currentLabel, [defaultLocale]: value }
+              : value;
+            return { ...item, label: nextLabel };
+          }),
+        },
+      };
+    });
+  }
+
   reorderNavLink(target: 'header' | 'footer', id: string, direction: -1 | 1): void {
     this.updateBootstrap(current => {
       const list = current.navigation[target];

src/app/features/project-editor/sections/navigation-section.component.ts

@@ -46,7 +46,7 @@ export class ProjectEditorNavigationSectionComponent {
   }
 
   updateLabel(target: 'header' | 'footer', id: string, value: string): void {
-    this.facade.updateNavLink(target, id, { label: value });
+    this.facade.updateNavLinkLabel(target, id, value);
   }
 
   updateRoute(target: 'header' | 'footer', id: string, value: string): void {