fix(project-editor): editable nav link labels per locale, add LocaleTabs

- navigation-section: add LocaleTabs; label input now reads/writes the
  active locale's translation via a new editableLabel() helper instead of
  always the default locale. facade.updateNavLinkLabel() gained an optional
  locale param (defaults to current default locale, so existing callers
  are unaffected) and correctly promotes a plain-string label into a
  per-locale map when writing a non-default locale.
- languages-section: wrap in SectionCard, add LocaleTabs for consistency.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -1,7 +1,7 @@
 import { Injectable, computed, inject, signal } from '@angular/core';
 import { take } from 'rxjs/operators';
 import { BootstrapConfig, DEFAULT_CATALOG_CONFIG, DEFAULT_HEADER_CONFIG, DEFAULT_PRODUCT_PAGE_CONFIG, DEFAULT_USER_EXPERIENCE_CONFIG } from '../../../shared/models/config';
-import { NavigationItemConfig } from '../../../shared/models/config';
+import { NavigationItemConfig, NavigationLocalizedText } from '../../../shared/models/config';
 import { ConfigService } from '../../../core/config/config.service';
 import { ProjectEditorIoService } from '../services/project-editor-io.service';
 import { ProjectEditorPreviewService } from '../services/project-editor-preview.service';
@@ -242,13 +242,14 @@ export class ProjectEditorFacade {
    * 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 {
+  updateNavLinkLabel(target: 'header' | 'footer', id: string, value: string, locale?: string): void {
     this.updateBootstrap(current => {
       const list = current.navigation[target];
       if (!this.isFlatNavList(list)) {
         return current;
       }
       const defaultLocale = current.localization.defaultLocale ?? 'en';
+      const targetLocale = locale ?? defaultLocale;
       return {
         ...current,
         navigation: {
@@ -258,9 +259,14 @@ export class ProjectEditorFacade {
               return item;
             }
             const currentLabel = item.label;
-            const nextLabel = currentLabel && typeof currentLabel === 'object'
-              ? { ...currentLabel, [defaultLocale]: value }
-              : value;
+            let nextLabel: string | NavigationLocalizedText;
+            if (currentLabel && typeof currentLabel === 'object') {
+              nextLabel = { ...currentLabel, [targetLocale]: value };
+            } else if (targetLocale === defaultLocale) {
+              nextLabel = value;
+            } else {
+              nextLabel = { [defaultLocale]: currentLabel ?? '', [targetLocale]: value };
+            }
             return { ...item, label: nextLabel };
           }),
         },