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/transcode/encode.go

@@ -107,6 +107,52 @@ func encodeWallet(w backendclient.WalletResp) []byte {
 	return b.FinishedBytes()
 }
 
+// encodeCatalog builds a Catalog payload: the storefront products for the caller's context, each
+// with its atom composition (built bottom-up — every product's atoms vector is finished before
+// its product table, and every product before the products vector).
+func encodeCatalog(cat backendclient.CatalogResp) []byte {
+	b := flatbuffers.NewBuilder(256)
+	prods := make([]flatbuffers.UOffsetT, len(cat.Products))
+	for i, p := range cat.Products {
+		atoms := make([]flatbuffers.UOffsetT, len(p.Atoms))
+		for j, a := range p.Atoms {
+			at := b.CreateString(a.AtomType)
+			fb.CatalogAtomStart(b)
+			fb.CatalogAtomAddAtomType(b, at)
+			fb.CatalogAtomAddQuantity(b, int32(a.Quantity))
+			atoms[j] = fb.CatalogAtomEnd(b)
+		}
+		fb.CatalogProductStartAtomsVector(b, len(atoms))
+		for j := len(atoms) - 1; j >= 0; j-- {
+			b.PrependUOffsetT(atoms[j])
+		}
+		atomsVec := b.EndVector(len(atoms))
+
+		kind := b.CreateString(p.Kind)
+		pid := b.CreateString(p.ProductID)
+		title := b.CreateString(p.Title)
+		cur := b.CreateString(p.MoneyCurrency)
+		fb.CatalogProductStart(b)
+		fb.CatalogProductAddKind(b, kind)
+		fb.CatalogProductAddProductId(b, pid)
+		fb.CatalogProductAddTitle(b, title)
+		fb.CatalogProductAddChips(b, int32(p.Chips))
+		fb.CatalogProductAddMoneyAmount(b, p.MoneyAmount)
+		fb.CatalogProductAddMoneyCurrency(b, cur)
+		fb.CatalogProductAddAtoms(b, atomsVec)
+		prods[i] = fb.CatalogProductEnd(b)
+	}
+	fb.CatalogStartProductsVector(b, len(prods))
+	for i := len(prods) - 1; i >= 0; i-- {
+		b.PrependUOffsetT(prods[i])
+	}
+	prodVec := b.EndVector(len(prods))
+	fb.CatalogStart(b)
+	fb.CatalogAddProducts(b, prodVec)
+	b.Finish(fb.CatalogEnd(b))
+	return b.FinishedBytes()
+}
+
 func encodeProfile(p backendclient.ProfileResp) []byte {
 	b := flatbuffers.NewBuilder(192)
 	uid := b.CreateString(p.UserID)