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/i18n/translations.ts

@@ -500,6 +500,12 @@ export interface Translations {
     searchSuggestions: string;
     searchHistory: string;
     livePreview: string;
+    previewChangesTitle: string;
+    previewNoIssues: string;
+    previewNoChanges: string;
+    previewChangeField: string;
+    previewChangeBefore: string;
+    previewChangeAfter: string;
     languagesTab: string;
     addLanguage: string;
     removeLanguage: string;