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>

public/robots.txt

@@ -4,5 +4,17 @@ Allow: /
 # Block access to cart (user-specific data)
 Disallow: /cart
 
+# Block admin/backoffice and internal diagnostics
+Disallow: /*/backoffice
+Disallow: /*/edit
+Disallow: /*/project-editor
+Disallow: /__diagnostics
+
 # Crawl delay for polite crawling
 Crawl-delay: 1
+
+# Static baseline sitemap (home/catalog/search/wishlist/compare only) - see
+# public/sitemap.xml's own header comment for what this does and does not
+# cover (no per-tenant product/category/static-page URLs yet - needs a
+# backend/build-time generator, documented in docs/BACKEND.md).
+Sitemap: /sitemap.xml