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/i18n/translations.ts

@@ -533,6 +533,8 @@ export interface Translations {
     validationInvalidCss: string;
     validationDuplicateRoutes: string;
     validationInvalidWidgetConfig: string;
+    undo: string;
+    redo: string;
     statusDraft: string;
     statusPublished: string;
     unsavedChanges: string;