Update User, Cart models.

Added isEmpty (no items), setCity, dropItems to Cart.
The cart field, company, was made nullable.

Updated the user field from name to username and made it unique.

database/migrations/2023_06_15_134235_create_carts_table.php

@@ -14,7 +14,7 @@ return new class extends Migration
         Schema::create('carts', function (Blueprint $table) {
             $table->id();
             $table->unsignedBiginteger('city_id')->unsigned();
-            $table->unsignedBiginteger('company_id')->unsigned();
+            $table->unsignedBiginteger('company_id')->unsigned()->nullable();
             $table->unsignedBiginteger('user_id')->unsigned();
             $table->string('status'); // CART | DONE