feat(admin): complete category management

Sprint 20. Adds features/admin/categories/ (model, gateway interface +
local gateway, facade, list/editor pages), mirroring the admin/products
container/facade/service split.

- indented hierarchy view + native HTML5 drag-and-drop reorder
- visibility toggle, item counter, empty state, include-deleted filter
- editor: slug uniqueness validation, translations, SEO fields, breadcrumb
  preview, image via existing MediaPickerComponent
- soft delete/restore, blocked when a category has children or items
- draft/publish status + localStorage draft recovery (mirrors Project
  Editor autosave) + CanDeactivate unsaved-changes guard
- wired into app.routes.ts (replaces the categories coming-soon placeholder)
- docs/ADMIN.md + docs/BACKEND.md updated with the new gap detail

Not yet done: admin/products' category dropdown still reads from its own
AdminProductsGateway.loadCategories() rather than this gateway (Sprint 21).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/app.routes.ts

@@ -2,6 +2,7 @@ import { Routes } from '@angular/router';
 import { languageGuard } from './guards/language.guard';
 import { projectEditorDirtyGuard } from './features/project-editor/guards/project-editor-dirty.guard';
 import { adminAuthGuard } from './core/admin-auth/admin-auth.guard';
+import { adminCategoryDirtyGuard } from './features/admin/categories/guards/admin-category-dirty.guard';
 import { environment } from '../environments/environment';
 
 // Core routes (same across all brands)
@@ -73,8 +74,17 @@ const coreRoutes: Routes = [
       },
       {
         path: 'categories',
-        loadComponent: () => import('./features/backoffice/shared/backoffice-coming-soon-page.component').then(m => m.BackofficeComingSoonPageComponent),
-        data: { titleKey: 'dashboard.actionCategories' }
+        loadComponent: () => import('./features/admin/categories/pages/admin-categories-list-page.component').then(m => m.AdminCategoriesListPageComponent)
+      },
+      {
+        path: 'categories/create',
+        loadComponent: () => import('./features/admin/categories/pages/admin-category-editor-page.component').then(m => m.AdminCategoryEditorPageComponent),
+        canDeactivate: [adminCategoryDirtyGuard]
+      },
+      {
+        path: 'categories/:id/edit',
+        loadComponent: () => import('./features/admin/categories/pages/admin-category-editor-page.component').then(m => m.AdminCategoryEditorPageComponent),
+        canDeactivate: [adminCategoryDirtyGuard]
       },
       {
         path: 'static-pages',