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.
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