feat(static-pages): live device preview (desktop/tablet/mobile)

Milestone 5 of the Static Pages Module sprint.

- New StaticPagePreviewComponent: client-side, sanitized HTML preview at
  desktop/tablet(768px)/mobile(375px) widths, entirely without navigation or
  publish. Uses the same DomSanitizer.sanitize(SecurityContext.HTML, ...)
  pattern as the real storefront renderer (StaticPageComponent), so what
  authors preview here matches what will actually render live.
- Wired into static-pages-editor as a per-page collapsible "Preview" toggle,
  showing the default-locale (or first available) translation's html/title.
- i18n: staticPages.previewDesktop/Tablet/Mobile/Toggle in interface + en/ru/hy.

Gate: tsc --noEmit, npm test (57/57), arch:check, build all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/i18n/ru.ts

@@ -748,6 +748,10 @@ export const ru: Translations = {
     customTemplateLabel: 'Пользовательский шаблон',
     seoRobotsLabel: 'Robots',
     disabledBadge: 'Отключена',
+    previewDesktop: 'Десктоп',
+    previewTablet: 'Планшет',
+    previewMobile: 'Телефон',
+    previewToggle: 'Превью',
   },
   widgets: {
     unavailable: 'Виджет недоступен',