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>

docs/ADMIN.md

@@ -19,7 +19,9 @@ by the existing `adminAuthGuard` (`core/admin-auth/admin-auth.guard.ts`):
 /:lang/backoffice/products/create     -> AdminProductEditorPageComponent
 /:lang/backoffice/products/:id/edit   -> AdminProductEditorPageComponent
 /:lang/backoffice/products/:id/duplicate -> AdminProductEditorPageComponent
-/:lang/backoffice/categories    -> BackofficeComingSoonPageComponent
+/:lang/backoffice/categories             -> AdminCategoriesListPageComponent
+/:lang/backoffice/categories/create      -> AdminCategoryEditorPageComponent
+/:lang/backoffice/categories/:id/edit    -> AdminCategoryEditorPageComponent
 /:lang/backoffice/static-pages  -> BackofficeComingSoonPageComponent
 /:lang/backoffice/transactions  -> BackofficeComingSoonPageComponent
 /:lang/backoffice/orders        -> BackofficeComingSoonPageComponent
@@ -145,6 +147,56 @@ save" and "last publish" were indistinguishable after a publish. Added
 `ProjectEditorFacade`, set only inside `publish()`. `lastSavedAt` behavior is
 unchanged (still updated by both `save()` and `publish()`).
 
+## Sprint 20 - Category Management
+
+`features/admin/categories/` (model/gateway/facade/pages/components), same
+container/facade/service split as `admin/products` and `admin/dashboard`:
+
+```text
+src/app/features/admin/categories/
+  models/    admin-category.model.ts
+  services/  admin-categories-gateway.interface.ts
+             admin-categories-local.gateway.ts
+             admin-categories-form.factory.ts
+  facade/    admin-categories.facade.ts
+  guards/    admin-category-dirty.guard.ts
+  components/ admin-categories-list.component.*
+              admin-category-form.component.*
+  pages/     admin-categories-list-page.component.ts
+             admin-category-editor-page.component.ts
+```
+
+- **Hierarchy**: `AdminCategory.parentId` (nullable). List page renders a
+  flattened, indented tree (`AdminCategoriesFacade.rootCategories()` /
+  `childrenOf(id)`); the editor's parent `<select>` excludes the category
+  itself and its descendants to prevent cycles.
+- **Reordering**: native HTML5 drag-and-drop in
+  `admin-categories-list.component.ts` (`draggable`, `dragstart`/`drop`),
+  persists via `AdminCategoriesFacade.reorder()` which just rewrites `order`.
+- **Delete/restore**: soft delete (`deletedAt` timestamp). Blocked
+  client-side (`facade.canDelete()`) if the category has children or
+  `itemsCount > 0`; list has an "include deleted" filter with a Restore
+  action for soft-deleted rows.
+- **Draft/publish**: `status: 'draft' | 'published'`, set by the editor's
+  "Save Draft" vs "Publish" buttons (`AdminCategoriesFacade.saveDraft(publish)`).
+- **Local draft recovery + unsaved-changes guard**: every `updateDraft()`
+  call persists the in-progress category to `localStorage` under
+  `admin-category-draft:<id>` (via the existing `LocalStorageService`,
+  same pattern as Project Editor autosave); the editor reloads that draft
+  ahead of the saved value if present, and is cleared on save.
+  `adminCategoryDirtyGuard` (mirrors `projectEditorDirtyGuard`) blocks
+  navigation away from an unsaved edit with `window.confirm`.
+- **Image**: reuses the existing `MediaPickerComponent` (same one used by
+  Media Manager) rather than a free-text URL field.
+- **Seed data**: `AdminCategoriesLocalGateway` seeds its in-memory cache from
+  `BackofficeDataService.loadCategories()` (`CategoryCardConfig`, currently
+  flat/no hierarchy) - same swappable-provider pattern as
+  `AdminProductsLocalGateway`.
+- **Not yet wired**: `admin/products`' category `<select>` still uses
+  `AdminProductsGateway.loadCategories()` (its own `AdminProductCategoryOption`
+  seed), not `AdminCategoriesGateway` - unifying them is Sprint 21 scope
+  (`docs/SPRINT-PLAN.md`).
+
 ## Known gaps / backend needs
 
 - **Dashboard metrics endpoint.** Categories/Products counts are computed