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.

pkg/fbs/scrabble.fbs

@@ -872,3 +872,30 @@ table Wallet {
 table WalletBuyRequest {
   product_id:string;
 }
+
+// CatalogAtom is one atom line of a storefront product: the base value type it grants
+// ("chips"/"hints"/"noads_days"/"tournament") and how many of it the product carries.
+table CatalogAtom {
+  atom_type:string;
+  quantity:int;
+}
+
+// CatalogProduct is one storefront product projected for the caller's context. A "value" is
+// bought with chips (chips is its uniform price, money fields zero); a "pack" funds chips with
+// money (money_amount is its price in the context currency's minor units under money_currency,
+// chips zero). atoms lists what the product grants.
+table CatalogProduct {
+  kind:string;
+  product_id:string;
+  title:string;
+  chips:int;
+  money_amount:long;
+  money_currency:string;
+  atoms:[CatalogAtom];
+}
+
+// Catalog is the storefront: the products visible and purchasable in the caller's context — the
+// chip-priced values and the chip packs priced in the context's payment method.
+table Catalog {
+  products:[CatalogProduct];
+}