Add category picker to article view with draft-based editing

- Add setCategories API method for PUT /api/article/:sku/categories
- Create CategoryPickerDialog component for selecting categories
- Add CategoryPills component with add/remove functionality
- Category changes are now draft-based (like name editing)
- Save button commits both name and category changes together
- Delete icons hidden when only one category remains
- Plus button only visible in edit mode

Implements category assignment flow similar to name editing pattern.

src/api/index.js

@@ -261,6 +261,19 @@ export const article = {
       body: { eans },
     });
   },
+
+  /**
+   * PUT /api/article/:sku/categories
+   * Body: { categoryIds: number[] } — replace-all. Empty array clears all.
+   * Categories must belong to the article's company; ids must be unique.
+   * → 200 { article } (same shape as GET by SKU)
+   */
+  setCategories(sku, categoryIds) {
+    return http(`/api/article/${encodeURIComponent(sku)}/categories`, {
+      method: 'PUT',
+      body: { categoryIds },
+    });
+  },
 };
 
 /**