feat(project-editor): pre-publish change + validation preview

Milestone 5 of the Configuration Engine sprint.

- Facade: changeSummary computed - per modified field, before/after values
  (schema label + stringified diff vs originalBootstrap), reusing
  modifiedFields from M4.
- preview-section: new "changes since last publish" card ahead of the
  existing export/import/live-preview card - validation issue list
  (warning/error styled) plus a before/after change table. Reuses the
  existing, non-destructive ProjectEditorPreviewService.preview() call.
- i18n: previewChangesTitle/NoIssues/NoChanges/ChangeField/Before/After in
  interface + en/ru/hy.

Gate: tsc --noEmit, npm test (33/33), arch:check, build all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/project-editor/facade/project-editor.facade.ts

@@ -14,6 +14,15 @@ import { EditorSchemaService } from '../schema/editor-schema.service';
 import { History, commit as commitHistory, emptyHistory, redo as redoHistory, undo as undoHistory } from '../schema/history.util';
 
 const HISTORY_DEBOUNCE_MS = 300;
+
+export interface ChangeSummaryRow {
+  readonly fieldKey: string;
+  /** i18n key for the field's label. */
+  readonly labelKey: string;
+  readonly section?: ProjectEditorSectionId;
+  readonly before: string;
+  readonly after: string;
+}
 import { ProjectEditorDraftStorageService } from '../services/project-editor-draft-storage.service';
 import { EDITOR_SECTION_BOOTSTRAP_KEYS } from '../models/project-editor.model';
 
@@ -123,6 +132,37 @@ export class ProjectEditorFacade {
     }
     return sections;
   });
+  /** Per-field before/after diff vs the originally loaded/published config, for the pre-publish preview. */
+  readonly changeSummary = computed<ChangeSummaryRow[]>(() => {
+    const current = this.bootstrap();
+    const original = this.state().originalBootstrap;
+    if (!current || !original) {
+      return [];
+    }
+    const rows: ChangeSummaryRow[] = [];
+    for (const key of this.modifiedFields()) {
+      const field = this.schema.getField(key);
+      rows.push({
+        fieldKey: key,
+        labelKey: field?.labelKey ?? key,
+        section: field?.section,
+        before: this.stringifyValue(this.schema.getByPath(original, key)),
+        after: this.stringifyValue(this.schema.getByPath(current, key)),
+      });
+    }
+    return rows;
+  });
+
+  private stringifyValue(value: unknown): string {
+    if (value === null || value === undefined) {
+      return '';
+    }
+    if (typeof value === 'object') {
+      const json = JSON.stringify(value);
+      return json.length > 80 ? `${json.slice(0, 77)}…` : json;
+    }
+    return String(value);
+  }
 
   loadBootstrap(): void {
     this.configService.loadBootstrap(true).pipe(take(1)).subscribe({