feat(content-management): surface validation errors per-page

Replace global duplicate-slug/empty-title banners with per-page indicators:
a danger badge next to the offending page's heading, plus inline error text
on the specific pageId/slug app-form-field. Made ContentPageService.normalizeSlug
public (was private) so the component can match validation results to a given
page's normalized slug without duplicating the normalization logic.

Verified in browser: setting a duplicate slug live shows both the header
badge and the inline field error immediately.

Completes Sprint 3 (Static Page Generator): DRY cleanup, Design System
adoption, SEO field coverage, per-page validation UX.

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

@@ -4,13 +4,6 @@
     <app-button variant="primary" (click)="createPage()">{{ 'builder.createPage' | translate }}</app-button>
   </div>
 
-  @if (validation().duplicateSlugs.length > 0) {
-    <app-badge variant="danger">{{ 'staticPages.duplicateSlug' | translate }}</app-badge>
-  }
-  @if (validation().emptyTitles.length > 0) {
-    <app-badge variant="danger">{{ 'staticPages.emptyTitle' | translate }}</app-badge>
-  }
-
   @if (pages().length === 0) {
     <app-empty-state [title]="'builder.staticPages' | translate" [description]="'builder.createPage' | translate">
       <span slot="actions">
@@ -24,7 +17,15 @@
       <app-card padding="md">
         <div class="page-card">
           <div class="page-card__header">
-            <h3 class="page-card__title">{{ page.id }}</h3>
+            <h3 class="page-card__title">
+              {{ page.id }}
+              @if (hasDuplicateSlug(page)) {
+                <app-badge variant="danger">{{ 'staticPages.duplicateSlug' | translate }}</app-badge>
+              }
+              @if (hasEmptyTitle(page)) {
+                <app-badge variant="danger">{{ 'staticPages.emptyTitle' | translate }}</app-badge>
+              }
+            </h3>
             <div class="editor-actions">
               <app-button variant="ghost" size="sm" (click)="move(page.id, -1)">&uarr;</app-button>
               <app-button variant="ghost" size="sm" (click)="move(page.id, 1)">&darr;</app-button>
@@ -33,10 +34,10 @@
           </div>
 
           <div class="editor-grid three">
-            <app-form-field [label]="'builder.pageId' | translate">
+            <app-form-field [label]="'builder.pageId' | translate" [error]="hasEmptyTitle(page) ? ('staticPages.emptyTitle' | translate) : null">
               <app-input [ngModel]="page.id" (ngModelChange)="updatePage(page.id, { id: $event })" />
             </app-form-field>
-            <app-form-field [label]="'builder.slug' | translate">
+            <app-form-field [label]="'builder.slug' | translate" [error]="hasDuplicateSlug(page) ? ('staticPages.duplicateSlug' | translate) : null">
               <app-input [ngModel]="page.slug" (ngModelChange)="updatePage(page.id, { slug: $event })" />
             </app-form-field>
             <app-form-field [label]="'builder.iconLabel' | translate">