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/section.shared.scss

@@ -170,6 +170,57 @@ button.secondary {
   color: #991b1b;
 }
 
+.preview-validation {
+  margin: 0 0 12px;
+  padding-left: 1.25rem;
+  color: var(--warning-color, #b45309);
+
+  &.has-errors {
+    color: var(--error-color, #991b1b);
+  }
+
+  li.is-warning {
+    color: var(--warning-color, #b45309);
+  }
+
+  li.is-error {
+    color: var(--error-color, #991b1b);
+  }
+}
+
+.preview-validation-ok {
+  margin: 0 0 12px;
+  color: var(--text-secondary, #5f6e6a);
+}
+
+.preview-changes {
+  width: 100%;
+  border-collapse: collapse;
+  font-size: 0.9rem;
+
+  th,
+  td {
+    text-align: left;
+    padding: 8px 10px;
+    border-bottom: 1px solid var(--border-color, #d3dad9);
+  }
+
+  th {
+    color: var(--text-secondary, #5f6e6a);
+    font-weight: 600;
+  }
+
+  .preview-before {
+    color: var(--text-secondary, #5f6e6a);
+    text-decoration: line-through;
+  }
+
+  .preview-after {
+    color: var(--text-primary, #1e3c38);
+    font-weight: 600;
+  }
+}
+
 @media (max-width: 900px) {
   .editor-grid.two,
   .editor-grid.three,