feat(project-editor): add sticky save/publish bar with validation summary

Mounts a new ProjectEditorSaveBarComponent in the editor page that shows
draft/published status, unsaved-changes indicator, and validation issues,
wiring the Task 8 facade save()/publish()/dirty/status/validationIssues
signals to an actual UI for the first time.

src/app/features/project-editor/pages/project-editor-page.component.html

@@ -28,5 +28,6 @@
         @case ('preview') { <app-project-editor-preview-section /> }
       }
     </section>
+    <app-project-editor-save-bar />
   }
 </main>

src/app/features/project-editor/pages/project-editor-page.component.ts

@@ -17,6 +17,7 @@ import { ProjectEditorNavigationSectionComponent } from '../sections/navigation-
 import { ProjectEditorPreviewSectionComponent } from '../sections/preview-section.component';
 import { TranslatePipe } from '../../../i18n/translate.pipe';
 import { StaticPagesEditorComponent } from '../../content-management/components/static-pages-editor.component';
+import { ProjectEditorSaveBarComponent } from '../components/save-bar/project-editor-save-bar.component';
 import { ProjectEditorSectionId } from '../models/project-editor.model';
 
 const KNOWN_SECTIONS: ProjectEditorSectionId[] = [
@@ -41,6 +42,7 @@ const KNOWN_SECTIONS: ProjectEditorSectionId[] = [
     ProjectEditorLanguagesSectionComponent,
     ProjectEditorNavigationSectionComponent,
     ProjectEditorPreviewSectionComponent,
+    ProjectEditorSaveBarComponent,
   ],
   templateUrl: './project-editor-page.component.html',
   styleUrls: ['./project-editor-page.component.scss'],