feat(platform): add typed contracts for optional Seller Management module

Architectural foundation only - no UI, no backend, no business logic.
Per ADR-001 (Platform -> Marketplace -> Seller hierarchy) and ADR-009
(feature flags / capability guards): Seller is an optional child scope
beneath a marketplace, not another tenant.

New:
- PlatformModulesConfig / SellerManagementModuleConfig
  (shared/models/config/platform-modules.model.ts) - the
  modules.sellerManagement.enabled contract, defaults to disabled
  (DEFAULT_PLATFORM_MODULES_CONFIG).
- SellerConfig (shared/models/config/seller.model.ts) - typed shape for
  the resolved seller scope, mirroring TenantConfig's fields at the
  subset a seller needs. Frontend never resolves this itself; it only
  reads what the backend already decided (same convention as tenant
  resolution, ADR-001).

Changed:
- BootstrapConfig gained two optional fields: modules?, seller?. Both
  absent by default - every existing marketplace's bootstrap response
  is untouched, TypeScript-checked backward compatible (all new fields
  optional, no existing field types changed).

tsc --noEmit clean. No component, facade, service, or route touched -
this commit is pure type contracts.

src/app/shared/models/config/bootstrap-config.model.ts

@@ -11,7 +11,9 @@ import { LocalizationConfig } from './localization.model';
 import { NavigationConfig } from './navigation.model';
 import { PageConfig } from './page.model';
 import { PermissionsConfig } from './permissions.model';
+import { PlatformModulesConfig } from './platform-modules.model';
 import { ProductPageConfig } from './product-page-config.model';
+import { SellerConfig } from './seller.model';
 import { SeoConfig } from './seo.model';
 import { StaticPagesConfig } from './static-page.model';
 import { TenantConfig } from './tenant.model';
@@ -42,4 +44,18 @@ export interface BootstrapConfig {
   pages: PageConfig[];
   staticPages?: StaticPagesConfig;
   widgetRegistry?: WidgetRegistryConfig;
+  /**
+   * Optional platform capability modules (e.g. Seller Management). Absent or
+   * `undefined` means every module is disabled - existing marketplaces that
+   * never send this field continue to behave exactly as today. See ADR-011.
+   */
+  modules?: PlatformModulesConfig;
+  /**
+   * Present only when `modules.sellerManagement.enabled` is true and the
+   * resolved request scope is beneath a specific seller. Absent for every
+   * marketplace-level request, including all existing marketplaces. The
+   * frontend never resolves this itself (ADR-001, ADR-011) - it only reads
+   * whatever the backend already decided.
+   */
+  seller?: SellerConfig;
 }