feat(payments): wallet screen with balances, benefits and storefront

Add the "Кошелёк" section to the settings hub: context-visible chip
balances, active benefits (no-ads term/forever, hints) and a storefront of
chip-priced values and money-priced chip packs. Guests have no wallet; the
Google Play build hides the money purchases behind a RuStore stub; a web
purchase that would draw VK/Telegram chips warns first.

Add the catalog read path the storefront needs — a context-projected
GET /api/v1/user/wallet/catalog (payments service + store, gateway op
wallet.catalog, FBS Catalog/CatalogProduct/CatalogAtom, client decode) —
plus the client leg for the existing wallet.get/buy ops. Value spends reuse
the existing spend path; the chip-pack purchase (money order flow) arrives
with payment intake, so its action is a disabled placeholder for now.

Covered by Go unit (catalog projection) + integration (/wallet/catalog over
Postgres), vitest (formatting, spendable selection, web-spend warning, GP
flag, codec + gateway encode round-trips) and Playwright mock e2e (render,
guest-hidden, GP stub, warning) on Chromium + WebKit.

backend/internal/payments/store_catalog.go

@@ -0,0 +1,89 @@
+package payments
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/go-jet/jet/v2/postgres"
+	"github.com/google/uuid"
+
+	"scrabble/backend/internal/postgres/jet/payments/model"
+	"scrabble/backend/internal/postgres/jet/payments/table"
+)
+
+// atomQty is one atom line of a product as loaded from the catalog: the atom type and quantity.
+type atomQty struct {
+	atomType string
+	quantity int
+}
+
+// priceRow is one price of a product as loaded from the catalog: the payment method (empty for a
+// value's CHIP price, stored with a NULL method) and the amount in that currency's minor units.
+type priceRow struct {
+	method   string
+	currency Currency
+	amount   int64
+}
+
+// catalogEntry is a raw active product loaded for the storefront: its atom composition and every
+// price row. [projectCatalog] turns it into the context-visible [CatalogProduct].
+type catalogEntry struct {
+	id     uuid.UUID
+	title  string
+	atoms  []atomQty
+	prices []priceRow
+}
+
+// loadCatalog reads every active product with its atoms and prices, ordered by creation, straight
+// from the catalog tables. The catalog is small and rarely edited (admin console only), so it is
+// read uncached — unlike the per-account balances/benefits the read cache fronts.
+func (s *Store) loadCatalog(ctx context.Context) ([]catalogEntry, error) {
+	var prods []model.Product
+	if err := postgres.SELECT(table.Product.AllColumns).
+		FROM(table.Product).
+		WHERE(table.Product.Active.IS_TRUE()).
+		ORDER_BY(table.Product.CreatedAt.ASC()).
+		QueryContext(ctx, s.db, &prods); err != nil {
+		return nil, fmt.Errorf("payments: load catalog products: %w", err)
+	}
+	if len(prods) == 0 {
+		return nil, nil
+	}
+	entries := make([]catalogEntry, len(prods))
+	index := make(map[uuid.UUID]int, len(prods))
+	for i, p := range prods {
+		entries[i] = catalogEntry{id: p.ProductID, title: p.Title}
+		index[p.ProductID] = i
+	}
+
+	var items []model.ProductItem
+	if err := postgres.SELECT(table.ProductItem.AllColumns).
+		FROM(table.ProductItem).
+		QueryContext(ctx, s.db, &items); err != nil {
+		return nil, fmt.Errorf("payments: load catalog items: %w", err)
+	}
+	for _, it := range items {
+		if i, ok := index[it.ProductID]; ok {
+			entries[i].atoms = append(entries[i].atoms, atomQty{atomType: it.AtomType, quantity: int(it.Quantity)})
+		}
+	}
+
+	var prices []model.ProductPrice
+	if err := postgres.SELECT(table.ProductPrice.AllColumns).
+		FROM(table.ProductPrice).
+		QueryContext(ctx, s.db, &prices); err != nil {
+		return nil, fmt.Errorf("payments: load catalog prices: %w", err)
+	}
+	for _, pr := range prices {
+		i, ok := index[pr.ProductID]
+		if !ok {
+			continue // a price for a deactivated product — not in the storefront
+		}
+		method := ""
+		if pr.Method != nil {
+			method = *pr.Method
+		}
+		entries[i].prices = append(entries[i].prices, priceRow{method: method, currency: Currency(pr.Currency), amount: pr.Amount})
+	}
+	return entries, nil
+}