feat: add brands and variant categories management

- Implemented CRUD operations for brands and variant categories in the API.
- Created UI components for managing brands and variant categories, including listing, creating, editing, and deleting.
- Enhanced the sidebar navigation to include links for brands and variant categories.
- Updated the categories API to support pagination and filtering.
- Added validation for brand and variant category names.
- Integrated toast notifications for user feedback on actions.

docs/20-FRONTEND.md

@@ -125,6 +125,11 @@ Each screen calls the endpoints in `11-BACKEND-PHASE1.md`. System steps (blue) a
 | Adjustment | Adjustment | `POST /stock-adjustments` |
 | Count | Count | `POST /stock-counts`, `PUT /stock-counts/{id}/counts`, `POST /stock-counts/{id}/post` |
 
+### 2.2 Master data screens (supporting, outside the core flow)
+Vendors, Items, Categories, UOM, Warehouses, Brands, and Variant Categories are supporting master-data CRUD screens the flow above depends on but doesn't itself route through, so they're intentionally absent from the diagram/table. List screens follow one pagination convention: `page`/`pageSize`/`q`/`sortOrder` params, page size 5, debounced search, Previous/Next controls.
+
+**Brand** (`app/dashboard/products/brands`) and **Variant Category** (`app/dashboard/products/variants`) are UI-only additions with no corresponding endpoint in `11-BACKEND-PHASE1.md` — Item's `brandId` is built the same way. The Item variant builder on `/dashboard/products/new` reads the Variant Category list live: checking a category (Color, Size, or any custom one added inline from that same page) reveals a value-entry section for it, and one Item is auto-created per combination across however many categories are checked, with an auto-generated SKU. Flag Brand/Variant Category to whoever owns the backend contract if they should become real entities rather than staying frontend-only; see `Frontend/PROGRESS.md` (2026-07-15 entries) for the full rationale and discarded design iterations.
+
 ---
 
 ## 3. Validation posture (read carefully)