feat: Phase 6 frontend - server-cart core, mock-gateway backed (scoped)

core/cart against docs/backend/PHASE-6-CART-CHECKOUT-CONTRACT.md §2-5:
ServerCart/ServerCartLine/CheckoutSession/DeliveryOption models + gateway/
token, in-memory mock implementation.

Deliberately does not touch pages/cart/cart.component.ts or
services/cart.service.ts (the live localStorage/Telegram-CloudStorage
cart) or features/website/checkout/ (still an empty directory) - same
judgment as Phases 1/3/5: this is real money/payment-adjacent flow and
deserves a dedicated, verified rewiring pass once a real backend exists,
not a bundled swap alongside nine other phases.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/core/cart/models/server-cart.model.ts

@@ -0,0 +1,35 @@
+/** Per docs/backend/PHASE-6-CART-CHECKOUT-CONTRACT.md §2. */
+export interface ServerCart {
+  id: string;
+  marketplaceId: string;
+  customerId?: string;
+  sessionToken?: string;
+  createdAt: string;
+  expiresAt: string;
+}
+
+export interface ServerCartLine {
+  id: string;
+  cartId: string;
+  offerId: string;
+  qty: number;
+  addedAt: string;
+  priceChanged?: boolean;
+}
+
+export interface DeliveryOption {
+  id: string;
+  marketplaceId: string;
+  label: string;
+  type: 'pickup' | 'courier' | 'digital';
+}
+
+export interface CheckoutSession {
+  id: string;
+  cartId: string;
+  customerContact: { email?: string; phone?: string; verified: boolean };
+  deliveryOptionId: string;
+  status: 'open' | 'confirmed' | 'expired';
+  createdAt: string;
+  expiresAt: string;
+}

src/app/core/cart/services/server-cart-gateway.interface.ts

@@ -0,0 +1,11 @@
+import { Observable } from 'rxjs';
+import { CheckoutSession, ServerCart, ServerCartLine } from '../models/server-cart.model';
+
+/** Per docs/backend/PHASE-6-CART-CHECKOUT-CONTRACT.md §3, §5. */
+export interface ServerCartGateway {
+  getCart(): Observable<{ cart: ServerCart; lines: ServerCartLine[] }>;
+  addLine(offerId: string, qty: number): Observable<ServerCartLine>;
+  updateLine(lineId: string, qty: number): Observable<ServerCartLine>;
+  removeLine(lineId: string): Observable<void>;
+  startCheckout(deliveryOptionId: string, currency: string): Observable<CheckoutSession>;
+}

src/app/core/cart/services/server-cart-gateway.token.ts

@@ -0,0 +1,9 @@
+import { InjectionToken, inject } from '@angular/core';
+import { ServerCartGateway } from './server-cart-gateway.interface';
+import { ServerCartLocalGateway } from './server-cart-local.gateway';
+
+/** Swap point for docs/backend/PHASE-6-CART-CHECKOUT-CONTRACT.md §3, §5. */
+export const SERVER_CART_GATEWAY = new InjectionToken<ServerCartGateway>('SERVER_CART_GATEWAY', {
+  providedIn: 'root',
+  factory: () => inject(ServerCartLocalGateway),
+});

src/app/core/cart/services/server-cart-local.gateway.ts

@@ -0,0 +1,66 @@
+import { Injectable } from '@angular/core';
+import { Observable, of } from 'rxjs';
+import { CheckoutSession, ServerCart, ServerCartLine } from '../models/server-cart.model';
+import { ServerCartGateway } from './server-cart-gateway.interface';
+
+const CART_TTL_MS = 30 * 24 * 60 * 60 * 1000;
+
+/**
+ * In-memory stand-in for the server cart. The LIVE cart today is
+ * localStorage/Telegram-CloudStorage backed (pages/cart/cart.component.ts,
+ * services/cart.service.ts) and deliberately untouched by this module - see
+ * docs/backend/PHASE-6-CART-CHECKOUT-CONTRACT.md for why swapping that live
+ * payment-adjacent flow needs its own dedicated, verified pass rather than
+ * a bundled mock-data rewire.
+ */
+@Injectable({ providedIn: 'root' })
+export class ServerCartLocalGateway implements ServerCartGateway {
+  private cart: ServerCart = {
+    id: 'cart_local',
+    marketplaceId: 'default',
+    sessionToken: 'local-session',
+    createdAt: new Date().toISOString(),
+    expiresAt: new Date(Date.now() + CART_TTL_MS).toISOString(),
+  };
+  private lines: ServerCartLine[] = [];
+
+  getCart(): Observable<{ cart: ServerCart; lines: ServerCartLine[] }> {
+    return of({ cart: this.cart, lines: this.lines });
+  }
+
+  addLine(offerId: string, qty: number): Observable<ServerCartLine> {
+    const existing = this.lines.find(l => l.offerId === offerId);
+    if (existing) {
+      existing.qty += qty;
+      return of(existing);
+    }
+    const line: ServerCartLine = { id: `line_${Date.now()}`, cartId: this.cart.id, offerId, qty, addedAt: new Date().toISOString() };
+    this.lines.push(line);
+    return of(line);
+  }
+
+  updateLine(lineId: string, qty: number): Observable<ServerCartLine> {
+    const line = this.lines.find(l => l.id === lineId);
+    if (line) {
+      line.qty = qty;
+    }
+    return of(line as ServerCartLine);
+  }
+
+  removeLine(lineId: string): Observable<void> {
+    this.lines = this.lines.filter(l => l.id !== lineId);
+    return of(void 0);
+  }
+
+  startCheckout(deliveryOptionId: string, _currency: string): Observable<CheckoutSession> {
+    return of({
+      id: `chk_${Date.now()}`,
+      cartId: this.cart.id,
+      customerContact: { verified: false },
+      deliveryOptionId,
+      status: 'open',
+      createdAt: new Date().toISOString(),
+      expiresAt: new Date(Date.now() + 15 * 60 * 1000).toISOString(),
+    });
+  }
+}