feat(project-editor): per-field inline validation for languages, homepage, widgets, navigation, static-pages

Extends the fieldError() wiring pattern (already used in theme/general/branding)
to the remaining sections that have matching ProjectValidator fieldKeys:
- languages: localization.supportedLocales (no-languages, missing-translations)
- homepage: pages (empty-homepage, missing-widget, duplicate-routes)
- widgets: pages (invalid-widget-config)
- navigation: navigation.header (duplicate-nav-links)
- static-pages: staticPages (duplicate-slugs, invalid-css)

Header/footer/features sections have no matching validator issues today,
so nothing to wire there yet.

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

src/app/features/content-management/components/static-pages-editor.component.html

@@ -3,6 +3,9 @@
     <h2>{{ 'builder.staticPages' | translate }}</h2>
     <app-button variant="primary" (click)="createPage()">{{ 'builder.createPage' | translate }}</app-button>
   </div>
+  @if (fieldError('staticPages'); as msg) {
+    <p class="editor-error">{{ msg }}</p>
+  }
 
   <div class="editor-grid three static-pages-toolbar">
     <app-form-field [label]="'staticPages.searchPlaceholder' | translate">