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/features/admin/shell/admin-nav.model.ts

@@ -37,6 +37,8 @@ export const ADMIN_NAV_PRIMARY: AdminNavEntry[] = [
   { type: 'link', id: 'transactions', icon: 'creditCard', labelKey: 'adminShell.nav.transactions', path: ['transactions'] },
   { type: 'link', id: 'moderation', icon: 'star', labelKey: 'adminShell.nav.moderation', path: ['moderation'] },
   { type: 'link', id: 'reports', icon: 'chartBar', labelKey: 'adminShell.nav.reports', comingSoon: true },
+  { type: 'group', labelKey: 'adminShell.nav.partnersGroup' },
+  { type: 'link', id: 'seller-management', icon: 'store', labelKey: 'adminShell.nav.sellerManagement', path: ['partners', 'seller-management'] },
   { type: 'link', id: 'content', icon: 'edit', labelKey: 'adminShell.nav.content', absolutePath: ['edit', 'static-pages'] },
   { type: 'link', id: 'media', icon: 'images', labelKey: 'adminShell.nav.mediaLibrary', path: ['media'] },
   { type: 'link', id: 'marketplace-builder', icon: 'network', labelKey: 'adminShell.nav.marketplaceBuilder', absolutePath: ['edit'] },