Change MainItem to Category and Create AddCategory

Story Points: 2

# How?

Right now, the current inventory system isn't very user-friendly. After talking to Jeremy, it was decided that the MainItems seen on the Inventory page would be called Categories, housing the previously called "SubItems" as the actual items, each having a quantity associated.

This issue is to rename the MainItem to Category in all the appropriate places, and to add an AddCategory page that is separate from the AddItem page so that they are separate functions that are semantically different in the eyes of the user.

在程式碼庫中找出所有對 'MainItem' 的參照，這些參照可能位於前端元件、後端模型和資料庫結構中。將它們重新命名為 'Category'。此外，找到 AddItem 頁面以了解其結構，並建立一個獨立的 AddCategory 頁面。檢查現有的庫存相關測試，更新命名，並確保新頁面能正確整合。

由索引模型根據 Issue 內容生成。