feat(seo): tenant-driven meta tags, sitemap/robots, reduced-motion, docs

Completes the rest of Sprint 28 on top of the earlier admin-scoped
a11y/skeleton pass (576f260):

- SeoService.resetToDefaults() now reads real bootstrap.seo.default /
  branding instead of hardcoded placeholder text + a broken
  /og-image.jpg reference; auto-reapplies via an effect() whenever
  bootstrap (re)loads, same pattern as UiRuntimeFacade.
- New public/sitemap.xml (static baseline, documented per-tenant-dynamic
  limitation) + public/robots.txt Sitemap directive and admin/editor
  Disallow rules.
- Global prefers-reduced-motion override in styles.scss covering every
  existing hover-transform/fade-in/shimmer animation in one place.
- New adminProducts/adminUsers/adminMonitoring/adminAnalytics
  empty-state i18n keys (en/ru/hy) for this sprint's skeleton/empty-state
  consistency fixes.
- docs/KNOWN-ISSUES.md: logged a newly-found, much larger pre-existing
  gap (~178 missing adminXxx.* i18n keys across the whole admin
  backoffice) - deferred to Sprint 29's translation validation, not
  fixed here.
- docs/BACKEND.md: new item 17 (sitemap generation gap).
- docs/ADMIN.md, docs/SPRINT-PLAN.md: rewritten Sprint 28 sections to
  describe the full, combined scope (both commits) instead of the
  earlier admin-only framing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/i18n/en.ts

@@ -862,4 +862,21 @@ export const en: Translations = {
     emptyTitle: 'No files yet',
     emptyDescription: 'Upload an image or document to get started.',
   },
+  adminProducts: {
+    emptyTitle: 'No products found',
+    emptyDescription: 'Try adjusting your filters, or create a new product.',
+  },
+  adminUsers: {
+    emptyTitle: 'No users found',
+    emptyDescription: 'Invite a teammate to get started.',
+    invitationsEmptyTitle: 'No pending invitations',
+  },
+  adminMonitoring: {
+    eventsEmptyTitle: 'No events found',
+    eventsEmptyDescription: 'Try a different category filter or search term.',
+  },
+  adminAnalytics: {
+    topProductsEmptyTitle: 'No product sales in this period',
+    topProductsEmptyDescription: 'Try a wider date range.',
+  },
 };