feat(favorites): keep customer favourites on the server, as legacy did (FAV-01)

Both apps kept favourites in memory only, and the API had no route, so
the 874 favourites imported from legacy major_favorites never showed.

- GET customer/me/favorites, POST customer/me/favorites/toggle
- apps load them on sign-in, clear on sign-out, toggle optimistically
  and roll back on failure

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/user-app/src/api/client.ts

@@ -336,6 +336,14 @@ export const api = {
   getAccountingBalance: () => request<AccountingBalance>('/customer/me/accounting-balance'),
   getMyOrders: () => request<OrderDetail[]>('/customer/me/orders'),
   getMyLedgerEntries: () => request<LedgerEntrySummary[]>('/customer/me/ledger-entries'),
+  /** Favourite product ids, newest first — legacy get-favorites (FAV-01). */
+  getFavorites: () => request<{ ids: string[] }>('/customer/me/favorites'),
+  /** Legacy add-to-fav: adds or removes; answers the new state. */
+  toggleFavorite: (productId: string) =>
+    request<{ favorite: boolean }>('/customer/me/favorites/toggle', {
+      method: 'POST',
+      body: JSON.stringify({ productId }),
+    }),
 
   createOrder: (dto: CreateOrderDto) => request<OrderSummary>('/orders', { method: 'POST', body: JSON.stringify(dto) }),
   getLatestGoldPrice: (karatPerMille = 750) => request<GoldPriceQuote>(`/pricing/latest?karatPerMille=${karatPerMille}`),