feat(project-editor): session undo/redo and modified-field tracking

Milestone 4 of the Configuration Engine sprint.

- Add schema/history.util: pure undo/redo reducer (commit/undo/redo, depth cap)
  with full spec coverage.
- Facade: debounced snapshot history (~300ms coalesce so a typing burst = one
  undo step); undo()/redo() route through the draft-save path so autosave never
  desyncs; canUndo/canRedo; history cleared on load/publish/resetDraft.
  modifiedFields (schema-diff vs original) + modifiedSections computeds.
- save-bar: Undo/Redo buttons. Page: Ctrl/Cmd+Z / Shift+Z / Y shortcuts
  (skipped while a text field is focused so native text undo is preserved);
  beforeunload guard already present.
- nav: amber modified-field dot per section (when no blocking badge).
- i18n: builder.undo / builder.redo 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/components/project-editor-nav.component.html

@@ -5,6 +5,6 @@
       routerLinkActive="active"
       [ariaCurrentWhenActive]="'page'"
       class="editor-nav-link"
-    >{{ section.label | translate }}@if (issueCount(section.id); as count) {<span class="editor-nav-badge" [attr.aria-label]="count + ' ' + ('builder.title' | translate)">{{ count }}</span>}</a>
+    >{{ section.label | translate }}@if (issueCount(section.id); as count) {<span class="editor-nav-badge" [attr.aria-label]="count + ' ' + ('builder.title' | translate)">{{ count }}</span>} @else if (isModified(section.id)) {<span class="editor-nav-dot" [attr.aria-label]="'builder.unsavedChanges' | translate"></span>}</a>
   }
 </nav>

src/app/features/project-editor/components/project-editor-nav.component.scss

@@ -58,6 +58,20 @@
   color: var(--error-color, #ef4444);
 }
 
+.editor-nav-dot {
+  display: inline-block;
+  width: 6px;
+  height: 6px;
+  margin-left: 6px;
+  border-radius: 999px;
+  background: var(--warning-color, #f59e0b);
+  vertical-align: middle;
+}
+
+.editor-nav-link.active .editor-nav-dot {
+  background: #fff;
+}
+
 @media (prefers-reduced-motion: reduce) {
   .editor-nav-link {
     transition: none;

src/app/features/project-editor/components/project-editor-nav.component.ts

@@ -16,12 +16,18 @@ import { ProjectEditorFacade } from '../facade/project-editor.facade';
 export class ProjectEditorNavComponent {
   private readonly facade = inject(ProjectEditorFacade);
   private readonly issuesBySection = this.facade.issuesBySection;
+  private readonly modifiedSections = this.facade.modifiedSections;
 
   /** Count of blocking issues in a section, for the nav badge. */
   issueCount(sectionId: ProjectEditorSectionId): number {
     return this.issuesBySection().get(sectionId) ?? 0;
   }
 
+  /** Whether a section has unsaved modified fields, for the nav dot. */
+  isModified(sectionId: ProjectEditorSectionId): boolean {
+    return this.modifiedSections().has(sectionId);
+  }
+
   readonly sections: Array<{ id: ProjectEditorSectionId; label: string }> = [
     { id: 'general', label: 'builder.general' },
     { id: 'branding', label: 'builder.branding' },

src/app/features/project-editor/components/save-bar/project-editor-save-bar.component.html

@@ -21,6 +21,8 @@
     }
   </div>
   <div class="project-editor-save-bar-actions">
+    <app-button variant="ghost" size="sm" [disabled]="!canUndo()" (click)="undo()">{{ 'builder.undo' | translate }}</app-button>
+    <app-button variant="ghost" size="sm" [disabled]="!canRedo()" (click)="redo()">{{ 'builder.redo' | translate }}</app-button>
     <app-button variant="danger" size="sm" (click)="resetDraft()">{{ 'builder.resetDraft' | translate }}</app-button>
     <app-button variant="secondary" size="sm" (click)="save()">{{ 'builder.save' | translate }}</app-button>
     <app-button variant="primary" size="sm" [disabled]="hasBlockingIssues()" (click)="publish()">{{ 'builder.publish' | translate }}</app-button>

src/app/features/project-editor/components/save-bar/project-editor-save-bar.component.ts

@@ -19,6 +19,16 @@ export class ProjectEditorSaveBarComponent {
   readonly status = this.facade.status;
   readonly issues = this.facade.validationIssues;
   readonly hasBlockingIssues = this.facade.hasBlockingIssues;
+  readonly canUndo = this.facade.canUndo;
+  readonly canRedo = this.facade.canRedo;
+
+  undo(): void {
+    this.facade.undo();
+  }
+
+  redo(): void {
+    this.facade.redo();
+  }
   readonly lastSavedAt = this.facade.lastSavedAt;
   readonly draftRestored = this.facade.draftRestored;