refactor(portal): move the header Back registry to shared data-access

WorkspaceBackNavigationService is root-provided signal state, which the
Nx ownership contract assigns to type:data-access; type:util takes new
pure helpers and contracts only. The service and its spec move to
@iptvnator/portal/shared/data-access, and the pure WorkspaceBackTarget
contract stays in @iptvnator/portal/shared/util.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/workspace/shell/feature/src/lib/workspace-shell/services/workspace-shell.facade.ts

@@ -3,9 +3,9 @@ import { toSignal } from '@angular/core/rxjs-interop';
 import { Router } from '@angular/router';
 import { TranslateService } from '@ngx-translate/core';
 import { startWith } from 'rxjs';
+import { WorkspaceBackNavigationService } from '@iptvnator/portal/shared/data-access';
 import {
     PORTAL_EXTERNAL_PLAYBACK,
-    WorkspaceBackNavigationService,
     WorkspaceHeaderContextService,
     WorkspaceResolvedCommandItem,
 } from '@iptvnator/portal/shared/util';