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/ru.ts

@@ -535,6 +535,8 @@ export const ru: Translations = {
     validationInvalidCss: 'Статическая страница содержит недопустимый CSS.',
     validationDuplicateRoutes: 'Две или более страницы используют один и тот же маршрут.',
     validationInvalidWidgetConfig: 'У виджета отсутствует обязательное поле (id, type, version или props).',
+    undo: 'Отменить',
+    redo: 'Повторить',
     statusDraft: 'Черновик',
     statusPublished: 'Опубликовано',
     unsavedChanges: 'Есть несохранённые изменения',