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/seller-management/pages/admin-seller-management-page.component.scss

@@ -0,0 +1,100 @@
+.admin-seller-management-page {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 60vh;
+  padding: var(--space-lg, 1.5rem);
+}
+
+.admin-seller-management-page__icon {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 72px;
+  height: 72px;
+  border-radius: var(--radius-full, 999px);
+  background: color-mix(in srgb, var(--primary-color, #2f6e5d) 12%, transparent);
+  color: var(--primary-color, #2f6e5d);
+  margin-inline: auto;
+}
+
+.admin-seller-management-page__actions {
+  display: flex;
+  gap: var(--space-sm, 0.5rem);
+  flex-wrap: wrap;
+  justify-content: center;
+}
+
+.admin-seller-management-page__form {
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-md, 1rem);
+}
+
+.admin-seller-management-page__textarea {
+  width: 100%;
+  min-height: 96px;
+  resize: vertical;
+  border: 1px solid var(--border-color, #e4e4e7);
+  border-radius: var(--radius-md, 6px);
+  background: var(--bg-primary, #fff);
+  color: var(--text-primary, #1f322d);
+  font-family: inherit;
+  font-size: var(--font-size-md, 0.9375rem);
+  padding: var(--space-sm, 0.5rem) var(--space-md, 1rem);
+  transition: border-color var(--transition-fast, 120ms ease),
+    box-shadow var(--transition-fast, 120ms ease);
+
+  &::placeholder {
+    color: var(--text-secondary, #6b7280);
+  }
+
+  &:hover:not(:disabled) {
+    border-color: var(--primary-color, #2f6e5d);
+  }
+
+  &:focus-visible {
+    outline: none;
+    border-color: var(--primary-color, #2f6e5d);
+    box-shadow: 0 0 0 2px color-mix(in srgb, var(--primary-color, #2f6e5d) 25%, transparent);
+  }
+}
+
+.admin-seller-management-page__dialog-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: var(--space-sm, 0.5rem);
+  margin-top: var(--space-md, 1rem);
+}
+
+.admin-seller-management-page__feature-list {
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-xs, 0.375rem);
+  margin: var(--space-md, 1rem) 0 0;
+  padding-inline-start: var(--space-md, 1rem);
+  color: var(--text-primary, #1f322d);
+  font-size: var(--font-size-md, 0.9375rem);
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .admin-seller-management-page__textarea {
+    transition: none;
+  }
+}
+
+@media (max-width: 640px) {
+  .admin-seller-management-page {
+    min-height: auto;
+    padding: var(--space-md, 1rem);
+  }
+
+  .admin-seller-management-page__actions {
+    flex-direction: column;
+    width: 100%;
+
+    app-button {
+      width: 100%;
+    }
+  }
+}