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/sections/preview-section.component.html

@@ -1,3 +1,38 @@
+<app-section-card [title]="'builder.previewChangesTitle' | translate">
+  @if (issues().length > 0) {
+    <ul class="preview-validation" [class.has-errors]="hasBlockingIssues()">
+      @for (issue of issues(); track issue.code) {
+        <li [class.is-warning]="issue.severity === 'warning'" [class.is-error]="issue.severity === 'error'">{{ issue.message | translate }}</li>
+      }
+    </ul>
+  } @else {
+    <p class="preview-validation-ok">{{ 'builder.previewNoIssues' | translate }}</p>
+  }
+
+  @if (changeSummary().length > 0) {
+    <table class="preview-changes">
+      <thead>
+        <tr>
+          <th>{{ 'builder.previewChangeField' | translate }}</th>
+          <th>{{ 'builder.previewChangeBefore' | translate }}</th>
+          <th>{{ 'builder.previewChangeAfter' | translate }}</th>
+        </tr>
+      </thead>
+      <tbody>
+        @for (row of changeSummary(); track row.fieldKey) {
+          <tr>
+            <td>{{ row.labelKey | translate }}</td>
+            <td class="preview-before">{{ row.before || '—' }}</td>
+            <td class="preview-after">{{ row.after || '—' }}</td>
+          </tr>
+        }
+      </tbody>
+    </table>
+  } @else {
+    <p class="preview-validation-ok">{{ 'builder.previewNoChanges' | translate }}</p>
+  }
+</app-section-card>
+
 <app-section-card [title]="'builder.preview' | translate">
   <div class="editor-actions">
     <app-button variant="primary" (click)="preview()">{{ 'builder.livePreview' | translate }}</app-button>