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/en.ts

@@ -502,6 +502,12 @@ export const en: Translations = {
     searchSuggestions: 'Search Suggestions',
     searchHistory: 'Search History',
     livePreview: 'Live Preview',
+    previewChangesTitle: 'Changes since last publish',
+    previewNoIssues: 'No validation issues.',
+    previewNoChanges: 'No changes since the last publish.',
+    previewChangeField: 'Field',
+    previewChangeBefore: 'Before',
+    previewChangeAfter: 'After',
     languagesTab: 'Languages',
     addLanguage: 'Add Language',
     removeLanguage: 'Remove',