feat(item): add brand field to items

This commit introduces an optional 'brand' field to the item model, allowing for better organization and
differentiation.

- The Add/Edit modal now includes a form field to select or create a brand.
- The item list on the main page now displays the brand next to the item name.
- Name uniqueness validation is now brand-aware. An item name must be unique within the context of its brand.
- The `addItem` logic is centralized in the `useItemsStore` to handle ID and timestamp generation.

app/pages/index.vue

@@ -69,7 +69,7 @@
               />
               <div>
                 <p class="font-medium text-highlighted">
-                  {{ row.original.name }}
+                  <strong>{{ row.original.brand }}</strong> {{ row.original.name }}
                 </p>
               </div>
             </div>