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/server/handlers.go

@@ -67,8 +67,10 @@ func (s *Server) registerRoutes() {
 		u.GET("/block-status", s.handleBlockStatus)
 	}
 	if s.payments != nil {
-		// The wallet: the context-visible chip segments + benefits, and a chip spend on a value.
+		// The wallet: the context-visible chip segments + benefits, the storefront catalog, and a
+		// chip spend on a value.
 		u.GET("/wallet", s.handleWallet)
+		u.GET("/wallet/catalog", s.handleWalletCatalog)
 		u.POST("/wallet/buy", s.handleWalletBuy)
 	}
 	if s.links != nil {