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/sitemap.xml

@@ -0,0 +1,44 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  Static baseline sitemap - top-level, statically-known marketplace routes
+  only (home, catalog, search, wishlist, compare) for the site's default
+  locale segment ('ru', see app.routes.ts's `redirectTo: 'ru'` fallback).
+
+  Known limitation (documented, not faked): this is a multi-tenant,
+  config-driven platform (docs/ARCHITECTURE.md) - supported locales,
+  categories, products, and static pages are all resolved at runtime from
+  the tenant's bootstrap config, not enumerable at build time from the
+  frontend alone. A real per-tenant sitemap covering
+  /:lang/product/:id, /:lang/catalog/:categoryId, and /:lang/:staticPath
+  needs a backend/build-time job that reads the same bootstrap data source
+  and regenerates this file (or serves it dynamically) per tenant/domain -
+  see docs/BACKEND.md. Until that exists, this static file is a reasonable
+  floor, not the full picture.
+-->
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>/ru</loc>
+    <changefreq>daily</changefreq>
+    <priority>1.0</priority>
+  </url>
+  <url>
+    <loc>/ru/catalog</loc>
+    <changefreq>daily</changefreq>
+    <priority>0.9</priority>
+  </url>
+  <url>
+    <loc>/ru/search</loc>
+    <changefreq>weekly</changefreq>
+    <priority>0.5</priority>
+  </url>
+  <url>
+    <loc>/ru/wishlist</loc>
+    <changefreq>monthly</changefreq>
+    <priority>0.3</priority>
+  </url>
+  <url>
+    <loc>/ru/compare</loc>
+    <changefreq>monthly</changefreq>
+    <priority>0.3</priority>
+  </url>
+</urlset>