feat: add domain models

src/app/models/Chain.ts

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+export class Chain {
+  constructor(public name: string | null, public image: string | null, public id?: number) {}
+}

src/app/models/Establishment.ts

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+import { Chain } from './Chain';
+
+export class Establishment {
+  constructor(public chain: Chain, public address = '', public id?: number) {}
+}

src/app/models/Product.ts

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+export class Product {
+  constructor(
+    public barcode: string,
+    public name = '',
+    public image: string | null = null,
+    public id?: number
+  ) {}
+}

src/app/models/ProductEstablisment.ts

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+import { Establishment } from './Establishment';
+import { Product } from './Product';
+
+export class ProductEstablishment {
+  id?: number;
+  establishment: Establishment;
+  product: Product;
+
+  constructor(product: Product, establishment: Establishment, id?: number) {
+    this.id = id;
+    this.establishment = establishment;
+    this.product = product;
+  }
+}

src/app/models/Purchase.ts

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+import { Establishment } from './Establishment';
+import { Product } from './Product';
+
+export class Purchase {
+  #price: number;
+  constructor(
+    public establishment: Establishment,
+    public product: Product,
+    price = 0,
+    public quantity = 1,
+    public date = Date.now(),
+    public id?: number
+  ) {
+    this.#price = price * 100;
+  }
+
+  get price() {
+    return this.#price / 100;
+  }
+
+  set price(price: number) {
+    this.#price = price * 100;
+  }
+}