feat(admin): Seller Management Phase 1 UI - Partners section, empty state, request/learn-more dialogs

No backend, no CRUD, no API, no business logic - production-quality
UI only, built entirely from existing shared components (app-dialog,
app-empty-state, app-button, app-form-field, app-input, app-icon,
app-badge). Gated per ADR-011: reads
modules.sellerManagement.enabled from bootstrap (always false today,
no backend sets it) rather than hardcoding disabled state.

New:
- AdminSellerManagementPageComponent (features/admin/seller-management/
  pages/) - renders the specified empty state (title/description/
  Request Access + Learn More buttons) using existing shared/ui
  primitives only, no new UI infrastructure.
- Request Access dialog: Company/Email/Message form via
  app-form-field + app-input + a plain textarea (no dedicated
  textarea component exists yet, styled to match app-input's own
  tokens exactly). Submission is mocked (setTimeout), no API call.
  On submit: closes and opens a success dialog ("Thank you...").
- Learn More dialog: 6 capability bullets (seller dashboards,
  storefronts, permissions, analytics, product ownership, marketplace
  administration) under a "Coming Soon" badge.
- New admin nav group "Partners" > "Seller Management" link
  (admin-nav.model.ts), new route /backoffice/partners/seller-management
  (app.routes.ts), using the same loadComponent/breadcrumb pattern as
  every other admin route.

Translations: full en/ru/hy coverage, zero hardcoded strings - new
adminShell.nav.{partnersGroup,sellerManagement},
adminShell.pages.sellerManagement, and a new adminSellerManagement.*
namespace (emptyState/requestDialog/requestSuccessDialog/
learnMoreDialog) added to translations.ts (types) and all three
locale files.

Accessibility: inherited from app-dialog (role="dialog",
aria-modal, focus trap on Tab/Shift+Tab, Escape to close, focus
restored to trigger on close) - no new a11y code needed, reused as-is.

Responsive: existing --space-*/--font-size-* tokens throughout,
flex-wrap on button row, mobile breakpoint stacks actions full-width.

Verified live (ru locale, devBypassAdmin): nav group/link render
correctly, breadcrumb shows "Управление продавцами", empty state
copy matches spec exactly, Request Access dialog opens with all 3
fields + Cancel/Send Request, filled + submitted -> success dialog
with exact spec copy, Learn More dialog shows all 6 bullets + Coming
Soon badge, no console errors, verified again at 375px mobile
viewport. tsc --noEmit clean, ng build clean (pre-existing bundle-
budget warning only), arch:check (boundaries + cycles) clean.

src/app/i18n/en.ts

@@ -1990,6 +1990,8 @@ export const en: Translations = {
       moderation: 'Reviews & Moderation',
       reviews: 'Reviews',
       reports: 'Reports',
+      partnersGroup: 'Partners',
+      sellerManagement: 'Seller Management',
       content: 'Content',
       mediaLibrary: 'Media Library',
       marketplaceBuilder: 'Marketplace Builder',
@@ -2035,6 +2037,44 @@ export const en: Translations = {
       users: { title: 'Users', description: 'Admin team members and permissions' },
       monitoring: { title: 'Monitoring', description: 'Queue, webhook, and system event status' },
       analytics: { title: 'Analytics', description: 'Sales, products, and shopper behavior' },
+      sellerManagement: { title: 'Seller Management', description: 'Onboard and manage independent sellers on your marketplace' },
+    },
+  },
+  adminSellerManagement: {
+    emptyState: {
+      title: 'Seller Management',
+      description: 'Allow shopping malls, marketplaces and enterprise customers to onboard independent sellers while keeping centralized administration. This module is currently under development.',
+      requestAccess: 'Request Access',
+      learnMore: 'Learn More',
+    },
+    requestDialog: {
+      title: 'Request Access',
+      companyLabel: 'Company',
+      companyPlaceholder: 'Your company name',
+      emailLabel: 'Email',
+      emailPlaceholder: 'you@company.com',
+      messageLabel: 'Message',
+      messagePlaceholder: 'Tell us about your marketplace and seller needs',
+      cancel: 'Cancel',
+      send: 'Send Request',
+    },
+    requestSuccessDialog: {
+      title: 'Thank you',
+      message: 'Our team received your request. We will contact you once Seller Management becomes available.',
+      close: 'Close',
+    },
+    learnMoreDialog: {
+      title: 'Seller Management',
+      comingSoonBadge: 'Coming Soon',
+      features: {
+        sellerDashboards: 'Independent seller dashboards',
+        sellerStorefronts: 'Seller storefronts',
+        sellerPermissions: 'Seller permissions',
+        sellerAnalytics: 'Seller analytics',
+        productOwnership: 'Product ownership',
+        marketplaceAdministration: 'Marketplace administration',
+      },
+      close: 'Close',
     },
   },
 };