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/homepage-section.component.html

@@ -1,5 +1,8 @@
 @if (homePage()) {
 <app-section-card [title]="'builder.homepage' | translate">
+  @if (fieldError('pages'); as msg) {
+    <p class="editor-error">{{ msg }}</p>
+  }
   <div cdkDropList class="sortable-list" (cdkDropListDropped)="drop($event)">
     @for (section of sections(); track section.id) {
       <div class="sortable-item" cdkDrag>