fix(6): revert Item.id to optional to fix delete function

Made id required (breaking change) - reverted back to optional
This allows items without id to exist during lifecycle
Fixes 'Failed to delete item' error

The delete function was broken by forcing id to be required

frontend/lib/db.ts

@@ -1,7 +1,7 @@
 import Dexie, { Table } from 'dexie';
 
 export interface Item {
-  id: number;
+  id?: number;
   barcode: string;
   name: string;
   category: string;