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/project-editor/sections/navigation-section.component.html

@@ -1,5 +1,8 @@
 <app-section-card [title]="'builder.navigationHeader' | translate">
   <app-locale-tabs [locales]="supportedLocales()" [defaultLocale]="defaultLocale()" (activeLocale)="onActiveLocaleChange($event)" />
+  @if (fieldError('navigation.header'); as msg) {
+    <p class="editor-error">{{ msg }}</p>
+  }
 
   <div class="editor-actions">
     <app-button variant="primary" (click)="addLink('header')">{{ 'builder.addLink' | translate }}</app-button>