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.

gateway/internal/backendclient/api.go

@@ -396,6 +396,36 @@ func (c *Client) WalletBuy(ctx context.Context, userID, productID string) (Walle
 	return out, err
 }
 
+// CatalogAtomResp is one atom line of a storefront product: the value type it grants and quantity.
+type CatalogAtomResp struct {
+	AtomType string `json:"atom_type"`
+	Quantity int    `json:"quantity"`
+}
+
+// CatalogProductResp is one storefront product for the caller's context: a chip-priced value
+// (chips set) or a chip pack priced in the context method (money_amount + money_currency).
+type CatalogProductResp struct {
+	Kind          string            `json:"kind"`
+	ProductID     string            `json:"product_id"`
+	Title         string            `json:"title"`
+	Chips         int               `json:"chips"`
+	MoneyAmount   int64             `json:"money_amount"`
+	MoneyCurrency string            `json:"money_currency"`
+	Atoms         []CatalogAtomResp `json:"atoms"`
+}
+
+// CatalogResp is the storefront: the products visible and purchasable in the caller's context.
+type CatalogResp struct {
+	Products []CatalogProductResp `json:"products"`
+}
+
+// Catalog fetches the storefront in the caller's current execution context.
+func (c *Client) Catalog(ctx context.Context, userID string) (CatalogResp, error) {
+	var out CatalogResp
+	err := c.do(ctx, http.MethodGet, "/api/v1/user/wallet/catalog", userID, "", nil, &out)
+	return out, err
+}
+
 // BlockStatusResp is the caller's current manual-block state. Until is an RFC3339 UTC instant for
 // a temporary block, empty for a permanent one or when not blocked; Reason is resolved to the
 // account's language, empty when none was cited.